TRUST AGREEMENT Between BIOGEN, INC. And

Exhibit 4.6

TRUST AGREEMENT

Between

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BIOGEN, INC.

And

FIDELITY MANAGEMENT TRUST COMPANY

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BIOGEN SAVINGS PLAN

TRUST

Effective as of April 1, 1994

TABLE OF CONTENTS

SECTION PAGE
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1 TRUST.................................................................2

2 EXCLUSIVE BENEFIT AND REVERSION OF SPONSOR CONTRIBUTIONS..............2

3 DISBURSEMENTS.........................................................2
(a) Directions from Administrator
(b) Limitations

4 INVESTMENT OF TRUST...................................................3
(a) Selection of Investment Options
(b) Available Investment Options
(c) Participant Direction
(d) Mutual Funds
(e) Sponsor Stock
(f) Notes
(g) Reliance of Trustee Directions
(h) Trustee Powers

5 RECORDKEEPING AND ADMINISTRATIVE SERVICES TO BE PERFORMED............11
(a) General
(b) Accounts
(c) Inspection and Audit
(d) Effect of Plan Amendment
(e) Returns, Reports and Information

6 COMPENSATION AND EXPENSES............................................12

7 DIRECTIONS AND INDEMNIFICATION.......................................13
(a) Identity of Administrator and Named Fiduciary
(b) Directions from Administrator
(c) Directions from Named Fiduciary
(d) Co-Fiduciary Liability
(e) Indemnification
(f) Survival

8 RESIGNATION OR REMOVAL OF TRUSTEE....................................14
(a) Resignation
(b) Removal

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TABLE OF CONTENTS
(CONTINUED)

SECTION PAGE
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9 SUCCESSOR TRUSTEE...................................................14

(a) Appointment
(b) Acceptance
(c) Corporate Action

10 TERMINATION.........................................................15

11 RESIGNATION, REMOVAL, AND TERMINATION NOTICES.......................15

12 DURATION............................................................16

13 AMENDMENT OR MODIFICATION...........................................16

14 GENERAL.............................................................16

(a) Performance by Trustee, its Agents or Affiliates
(b) Entire Agreement
(c) Waiver
(d) Successors and Assigns
(e) Partial Invalidity
(f) Section Headings

15 GOVERNING LAW.......................................................17

(a) Massachusetts Law Controls
(b) Trust Agreement Controls

Schedules
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A. Recordkeeping and Administrative Services
B. Fee Schedule
C. Investment Options
D. Sponsor's Authorization Letter
E. Named Fiduciary's Authorization Letter
F. IRS Detamination Letter or Opinion of Counsel
G. Telephone Exchange Procedures

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3
4

TRUST AGREEMENT, effective as of the first day of April, 1994, between
BIOGEN, INC., a Massachusetts corporation, having an offce at Fourteen
Cambridge Center, Cambridge, MA 02142 (the "SPONSOR"), and FIDELITY MANAGEMENT
TRUST COMPANY, a Massachusetts trust company, having an off'ce at ▇▇ ▇▇▇▇▇▇▇▇▇▇
▇▇▇▇▇▇, ▇▇▇▇▇▇, ▇▇▇▇▇▇▇▇▇▇▇▇▇ ▇▇▇▇▇ (the "TRUSTEE").

WITNESSETH:

WHEREAS, the Sponsor is the sponsor of the Biogen Savings Plan (the
"PLAN"); and

WHEREAS, the Sponsor has established a trust to hold and invest plan assets
under the Plan for the exclusive benefit of participants in the Plan and their
beneficiaries; and

WHEREAS, pursuant to the provisions of the existing trust agreement, the
Sponsor wishes to replace the incumbent trustee and to appoint the Trustee
named above as successor trustee, and to amend and restate the existing trust
agreement with the terms of this instrument, all effective as of April 1, 1994;
and

WHEREAS, the Sponsor (the "NAMED FIDUCIARY") is a named fiduciary of the
Plan (within the meaning of section 402(a) of the Employee Retirement Income
Security Act of 1974, as amended (ERISA)); and

WHEREAS, the Trustee is willing to hold and invest the aforesaid plan
assets in trust among several investment options selected by the Named
Fiduciary; and

WHEREAS, the Sponsor wishes to have the Trustee perform certain ministerial
recordkeeping and administrative functions under the Plan; and

WHEREAS, the Sponsor (the "ADMINISTRATOR") is the administrator of the Plan
(within the meaning of section 3(16)(A) of ERISA); and

WHEREAS, the Trustee is willing to perform recordkeeping and administrative
services

for the Plan if the services are purely ministerial in nature and are provided
within a framework of plan provisions, guidelines and interpretations conveyed
in writing to the Trustee by the Administrator.

NOW, THEREFORE, in consideration of the foregoing premises and the mutual
covenants and agreements set forth below, the Sponsor and the Trustee agree as
follows:

SECTION 1. TRUST. The Sponsor hereby establishes the Biogen Savings Plan Trust
(the "Trust"), with the Trustee. The Trust shall consist of an initial
contribution of money and other property acceptable to the Trustee in its sole
discretion transferred from the predecessor trustee under the Plan, such
additional sums of money and Sponsor Stock (hereinafter defined) as shall from
time to time be delivered to the Trustee under the Plan, all investments made
therewith and proceeds thereof, and all earnings and profits thereon, less the
payments that are made by the Trustee as provided herein, without distinction
between principal and income. The Trustee hereby accepts the Trust on the terms
and conditions set forth in this Agreement. In accepting this Trust, the
Trustee shall be accountable for the assets received by it, subject to the
terms and conditions of this Agreement. Trustee shall render services hereunder
in accordance with applicable professional standards.

SECTION 2. EXCLUSIVE BENEFIT AND REVERSION OF SPONSOR CONTRIBUTIONS. Except as
provided under applicable law and the Plan, no part of the Trust may be used
for, or diverted to, purposes other than the exclusive benefit of the
participants in the Plan or their baneficiaries prior to the satisfaction of
all liabilities under the Plan with respect to the participants and their
baneficiaries.

SECTION 3. DISBURSEIRNENTS.

(a) DIRECTIONS FROM ADMINISTRATOR. The Trustee shall make disbursements
in the amounts and in the manner that the Administrator directs from time to
time in writing. The Trustee shall have no responsibility to ascertain any
direction's compliance with the terms of the Plan or of any applicable law or
the direction's effect for tax purposes or otherwise; nor shall the Trustee
have any responsibility to see to the application of any disbursement.

(b) LIMITATIONS. The Trustee shall not be required to make any
disbursement in

excess of the net realizable value of the assets of the Trust at the time of
the disbursement. The Trustee shall not be required to make any disbursement in
cash unless the Administrator has provided a written direction as to the assets
to be converted to cash for the purpose of making the disbursement.

SECTION 4. INVESTMENT OF TRUST.

(a) SELECTION OF INVESTMENT OPTIONS. The Trustee shall have no
responsibility for the selection of investment options under the Trust and
shall not render investment advice to any person in connection with the
selection of such options.

(b) AVAILABLE INVESTMENT OPTIONS. The Named Fiduciary shall direct the
Trustee as to what investment options: (i) the Trust shall be invested in
during the period beginning on the initial transfer of assets to the Trust and
ending on the completion of the reconciliation of participant records
(nparticipant recordkeeping reconciliation period,n) and (ii) the investment
options in which Plan participants may invest in, subject to the following
limitations. The Named Fiduciary may determine to offer as investment options
only (i) securities issued by the investment companies advised by Fidelity
Management & Research Company ("Mutual Funds"), (ii) equity securities issued
by the Sponsor or an aff~liate which are publicly-traded and which are
"qualifying employer securities" within the meaning of section 407(d)(5) of
ERISA ("Sponsor Stock"), (iii) notes evidencing loans to Plan participants in
accordance with the terms of the Plan, (iv) guaranteed investment contracts
chosen by the Trustee and (v) collective investment funds maintained by the
Trustee for qualified plans; provided, however, that the Trustee shall be
considered a fiduciary with investment discretion only with respect to Plan
assets that are invested in guaranteed investment contracts chosen by the
Trustee or in collective investment funds maintained by the Trustee for
qualified plans. The investment options initially selected by the Named
Fiduciary are identified on Schedules "A" and "C" attached hereto. The Named
Fiduciary may change or add additional investment options with the consent of
the Trustee and upon mutual amendment of this Trust Agreement and the Schedules
thereto to reflect such additions.

(c) PARTICIPANT DIRECTION. Subject to any limitations imposed by the
Sponsor (which limitations shall be noted on Schedules "A" and "C" hereto),
each Plan participant shall direct the Trustee in which investment option(s) to
invest the assets in the participant's individual

accounts. Such directions may be made by Plan participants by use of the
telephone exchange system maintained for such purposes by the Trustee or its
agent, in accordance with written Telephone Exchange Guidelines attached hereto
as Schedule "G". A participant shall be considered a named fiduciary of the
Plan under ERISA for purposes of using the telephone exchange system to provide
investment directions to the Trustee for the participant's individual account.
In the event that the Trustee fails to receive a proper direction, the assets
shall be invested in the securities of the Mutual Fund set forth for such
purpose on Schedule "C", until the Trustee receives a proper direction.

(d) MUTUAL FUNDS. The Sponsor hereby acknowledges that it has received
from the Trustee a copy of the prospectus for each Mutual Fund selected by the
Named Fiduciary as a Plan investment option. Trust investments in Mutual Funds
shall be subject to the following limitations:

(i) EXECUTION OF PURCHASES AND SALES. Purchases and sales of Mutual
Funds (other than for Exchanges) shall be made on the date on which the Trustee
receives from the Sponsor in good order all information and documentation
necessary to accurately effect such purchases and sales (or in the case of a
purchase, the subsequent date on which the Trustee has received a wire transfer
of funds necessary to make such purchase). Exchanges of Mutual Funds shall be
made in accordance with the Telephone Exchange Guidelines attached hereto as
Schedule "G".

(ii) VOTING. At the time of mailing of notice of each annual or
special stockholders' meeting of any Mutual Fund, the Trustee shall send a copy
of the notice and all proxy solicitation materials to each Plan participant who
has shares of the Mutual Fund credited to the participant's accounts, together
with a voting direction form for return to the Tmstee or its designee. The
participant shall have the right to direct the Trustee as to the manner in
which the Trustee is to vote the shares credited to the participant's accounts.
The Trustee shall vote the shares as directed by the participant. The Trustee
shall not vote shares for which it has received no directions from the
participant. During the participant recordkeeping reconciliation period, the
Sponsor shall have the right to direct the Trustee as to the manner in which
the Trustee is to vote the shares of the Mutual Funds in the Trust. With
respect to all rights other than the right to vote, the Trustee shall follow
the directions of the participant and if no such directions are

received, the directions of the Named Fiduciary. Except as provided in the
first sentence of this subsection (d)(ii), the Trustee shall have no duty to
solicit directions from participants or the Sponsor.

(e) SPONSOR STOCK. Trust investments in Sponsor Stock shall be
subject to the following limitations:

(i) ACQUISITION LIMIT. Pursuant to the Plan, the Trust shall be
invested in Sponsor Stock to the extent necessary to invest all employer
matching contributions accounts in Sponsor Stock. In order to accomplish this,
up to 99 percent of the Trust's assets may be invested in Sponsor Stock.

(ii) FIDUCIARY DUTY OF NAMED FIDUCIARY. The Named Fiduciary shall
monitor the suitability under the fiduciary duty rules of section 404(a)(1) of
ERISA (as modified by section 404(a)(2) of ER1SA) of acquiring and holding
Sponsor Stock. The Trustee shall not be liable for any loss, or by reason of
any breach, which arises from the directions of the Named Fiduciary with
respect to the acquisition and holding of Sponsor Stock, unless it is clear on
their face that the actions to be taken under those directions would be
prohibited by the foregoing fiduciary duty rules or would be contrary to the
terms of the Plan or this Agreement.

(iii) EXECUTION OF PURCHASES AND SALES.

(A) Purchases and sales of Sponsor Stock (other than for
exchanges) shall be made on the open market on the date on which the Trustee
receives from the Sponsor in good order all information and documentation
necessary to accurately effect such purchases and sales (or, in the case of
purchases, the subsequent date on which the Trustee has received a wire
transfer of the funds necessary to make such purchases). Exchanges of Sponsor
Stock are only permitted at the Sponsor's direction. Such general rules shall
not apply in the following circumstances:

(1) If without fault on its own part, the Trustee is unable
to determine the number of shares required to be purchased or sold on such day;
or

(2) If the Trustee is unable to purchase or sell the total
number of shares required to be purchased or sold on such day as a result of
market conditions; or

(3) If the Trustee is prohibited by the Securities and
Exchange Commission, the New York Stock Exchange, or any other regulatory body
from purchasing or selling any or all of the shares required to be purchased or
sold on such day.

In the event of the occurrence of the circumstances described in (1), (2), or
(3) above, the Trustee shall purchase or sell such shares as soon as possible
thereafter and shall determine the price of such purchases or sales to be the
average purchase or sales price of all such shares purchased or sold,
respectively. The Trustee may follow directions from the Named Fiduciary to
deviate from the above purchase and sale procedures provided that such
direction is made in writing by the Named Fiduciary.

(B) USE OF AN AFFILIATED BROKER. The Sponsor hereby authorizes
the Trustee to use Fidelity Brokerage Services, Inc. ("FBSI") to provide
brokerage services in connection with any purchase or sale of Sponsor Stock in
accordance with directions from Plan participants. FBSI shall execute such
directions directly or through its aff'liate, National Financial Services
Company ("NFSC"). The provision of brokerage services shall be subject to the
following:

(i) As consideration for such brokerage services, the
Sponsor agrees that FBSI shall be entitled to remuneration under this
authorization provision in the amount of five cents ($.05) commission on each
share of Sponsor Stock up to 10,000 shares in a singular transaction, four
cents ($.04) commission on each share of Sponsor Stock from 10,001 to 20,000
shares in a singular transaction, and three and one-half cents ($.035)
commission on each share of Sponsor Stock in excess of 20,000 shares in a
singular transaction. Any change in such remuneration may be made only by a
signed agreement between Sponsor and Trustee.

(ii) Following the procedures set forth in Department of
Labor Prohibited Transaction Class Exemption 86-128, the Trustee will provide
the Sponsor with the following documents: (1) a description of FBSI's brokerage
placement practices; (2) a copy of

PTCE 86-128; and (3) a form by which the Sponsor may terminate this
authorization to use a broker aff'liated with the Trustee. The Trustee will
provide the Sponsor with this termination form annually, as well as an annual
report which summarizes all securities transaction-related charges incurred by
the Plan, and the Plan's annualized turnover rate.

(iii) Any successor organization of FBSI, through
reorganization, consolidation, merger or similar transactions, shall, upon
consummation of such transaction, become the successor broker in accordance
with the terms of this authorization provision.

(iv) The Trustee and FBSI shall continue to rely on this
authorization provision until not)fied to the contrary. The Sponsor reserves
the right to terminate this authorization upon sixty (60) days written notice
to FBSI (or its successor) and the Trustee, in accordance with Section 11 of
this Agreement.

(v) SECURITIES LAW REPORTS. The Named Fiduciary shall be
responsible for filing all reports required under Federal or state securities
laws with respect to the Trust's ownership of Sponsor Stock, including, without
limitation, any reports required under section 13 or 16 of the Securities
Exchange Act of 1934, and shall immediately notify the Trustee in writing of
any requirement to stop purchases or sales of Sponsor Stock pending the filing
of any report. The Trustee shall provide to the Named Fiduciary such
information on the Trust's ownership of Sponsor Stock as the Named Fiduciary
may reasonably request in order to comply with Federal or state securities
laws.

(vi) VOTING AND TENDER OFFERS. Notwithstanding any other
provision of this Agreement the provisions of this Section shall govern the
voting and tendering of Sponsor Stock. The Sponsor, after consultation with the
Trustee, shall provide and pay for all printing, mailing, tabulation and other
costs associated with the voting and tendering of Sponsor Stock.

(A) VOTING.

(1) When the issuer of the Sponsor Stock prepares for any
annual meeting, the Sponsor shall notify the Trustee thirty (30) days in
advance of the intended

meeting date and shall cause a copy of all proxy solicitation materials to be
sent to the Trustee. Based on these materials the Trustee shall prepare a
voting instruction form. At the time of mailing of notice of each annual or
special stockholders' meeting of the issuer of the Sponsor Stock, the Sponsor
shall cause a copy of the notice and all proxy solicitation materials to be
sent to each Plan participant, together with the foregoing voting instruction
form to be returned to the Trustee or its designee. The form shall show the
number of full and fractional shares of Sponsor Stock credited to the
participant's accounts (as provided by the Trustee). The Sponsor shall provide
the Trustee with a copy of any materials provided to the participants and shall
certify to the Trustee that the materials have been mailed or otherwise sent to
participants.

(2) Each participant shall have the right to direct the
Trustee as to the manner in which the Trustee is to vote that number of shares
of Sponsor Stock credited to the participant's accounts (both vested and
unvested). Directions from a participant to the Trustee concerning the voting
of Sponsor Stock shall be communicated in writing, or by mailgram or similar
means. These directions shall be held in confidence by the Trustee and shall
not be divulged to the Sponsor, or any off'cer or employee thereof, or any
other person. Upon its receipt of the directions, the Trustee shall vote the
shares of Sponsor Stock as directed by the participant. The Trustee shall not
vote shares of Sponsor Stock credited to a participant's accounts for which it
has received no directions from the participant.

(B) TENDER OFFERS.

(1) Upon commencement of a tender offer for any securities
held in the Trust that are Sponsor Stock, the Sponsor shall notify each Plan
participant of the tender offer and utilize its best efforts to timely
distribute or cause to be distributed to the participant the same information
that is distributed to shareholders of the issuer of Sponsor Stock in
connection with the tender offer, and, after consulting with the Trustee, shall
provide and pay for a means by which the participant may direct the Trustee
whether or not to tender the Sponsor Stock credited to the participant's
accounts (both vested and unvested). The Sponsor shall provide the Trustee with
a copy of any material provided to the participants and shall certify to the
Trustee that the materials have been mailed or otherwise sent to participants.

(2) Each participant shall have the right to direct the
Trustee to tender or not to tender some or all of the shares of Sponsor Stock
credited to the participant's accounts (both vested and unvested). Directions
from a participant to the Trustee concerning the tender of Sponsor Stock shall
be communicated in writing, or by mailgram or such similar means as is agreed
upon by the Trustee and the Sponsor under the preceding paragraph. These
directions shall be held in confidence by the Trustee and shall not be divulged
to the Sponsor, or any officer or employee thereof, or any other person except
to the extent that the consequences of such directions are reflected in reports
regularly communicated to any such persons in the ordinary course of the
performance of the Trustee's services hereunder. The Trustee shall tender or
not tender shares of Sponsor Stock as directed by the participant. The Trustee
shall not tender shares of Sponsor Stock credited to a participant's accounts
for which it has received no directions from the participant.

(3) A participant who has directed the Trustee to tender
some or all of the shares of Sponsor Stock credited to the participant's
accounts may, at any time prior to the tender offer withdrawal date, direct the
Trustee to withdraw some or all of the tendered shares, and the Trustee shall
withdraw the directed number of shares from the tender offer prior to the
tender offer withdrawal deadline. A participant shall not be limited as to the
number of directions to tender or withdraw that the participant may give to the
Trustee.

(4) A direction by a participant to the Trustee to tender
shares of Sponsor Stock

(5) credited to the participant's accounts shall not be
considered a written election under the Plan by the participant to withdraw, or
have distributed, any or all of his withdrawable shares. The Trustee shall
credit to each account of the participant from which the tendered shares were
taken the proceeds received by the Trustee in exchange for the shares of
Sponsor Stock tendered from that account. Pending receipt of directions
(through the administrator) from the participant or the Named Fiduciary, as
provided in the Plan, as to which of the remaining investment options the
proceeds should be invested in, the Trustee shall invest the proceeds in the
Mutual Fund described in Schedule "C".

(vii) SHARES CREDITED. For all purposes of this Section,
the number of shares of Sponsor Stock deemed "credited" to a participant's
accounts shall be determined as of the last preceding valuation date.

(viii) GENERAL. With respect to all rights other than the
right to vote, the right to tender, and the right to withdraw shares previously
tendered, in the case of Sponsor Stock credited to a participant's accounts,
the Trustee shall follow the directions of the participant and if no such
directions are received, the directions of the Named Fiduciary. The Trustee
shall have no duty to solicit directions from participants.

(ix) CONVERSION. All provisions in this Section 4(e) shall
also apply to any securities received as a result of a conversion of Sponsor
Stock.

(f) NOTES. The Administrator shall act as the Trustee's agent for the
purpose of holding all trust investments in participant loan notes and related
documentation and as such shall (i) hold physical custody of and keep safe the
notes and other loan documents, (ii) collect and remit all principal and
interest payments to the Trustee, (iii) keep the proceeds of such loans
separate from the other assets of the Administrator and clearly identify such
assets as Plan assets, and (iv) cancel and surrender the notes and other loan
documentation when a loan has been paid in full. To originate a participant
loan, the Plan participant shall direct the Trustee as to the type of loan to
be made from the participant's individual account. Such directions shall be
made by Plan participants by use of the telephone exchange system maintained
for such purpose by the Trustee or its agent. The Trustee shall determine,
based on the current value of the participant's account, the amount available
for the loan. Based on the monthly interest rate supplied by the Sponsor in
accordance with the terms of the Plan, the Trustee shall advise the participant
of such interest rate, as well as the installment payment amounts. The Trustee
shall forward the loan document to the participant for execution and submission
for approval to the Administrator. The Administrator shall have the
responsibility for approving the loan and instructing the Trustee to send the
loan proceeds to the Administrator or to the participant if so directed by the
Administrator. In all cases, such instruction by the Administrator shall be
made within thirty (30) days of the participant's initial request (the
origination date).

(g) RELIANCE OF TRUSTEE ON DIRECTIONS.

(i) The Trustee shall not be liable for any loss, or by reason of
any breach, which arises from any participant's exercise or non-exercise of
rights under this Section 4 over the assets in the participant's accounts.

(ii) The Trustee shall not be liable for any loss, or by reason of
any breach, which arises from the Named Fiduciary's exercise or non-exercise of
rights under this Section 4, unless it was clear on their face that the actions
to be taken under the Named Fiduciary's directions were prohibited by the
fiduciary duty rules of section 404(a) of ERISA or were contrary to the terms
of the Plan or this Agreement.

(h) TRUSTEE POWERS. The Trustee shall have the following
powers and authority:

(i) Subject to paragraphs (b), (c), (d) and (e) of this Section 4,
to sell, exchange, convey, transfer, or otherwise dispose of any property held
in the Trust, by private contract or at public auction. No person dealing with
the Trustee shall be bound to see to the application of the purchase money or
other property delivered to the Trustee or to inquire into the validity,
expediency, or propriety of any such sale or other disposition.

(ii) Subject to paragraphs (b) and (c) of this Section 4, to invest
in guaranteed investment contracts and short term investments (including
interest bearing accounts with the Trustee or money market mutual funds advised
by aff~liates of the Trustee) and in collective investment funds maintained by
the Trustee for qualified plans, in which case the provisions of each
collective investment fund in which the Trust is invested shall be deemed
adopted by the Sponsor and the provisions thereof incorporated as a part of
this Trust as long as the fund remains exempt &om taxation under Sections
401(a) and 501(a) of the Internal Revenue Code of 1986, as amended.

(iii) To cause any securities or other property held as part of the
Trust to be registered in the Trustee's own name, in the name of one or more of
its nominees, or in the Trustee's account with the Depository Trust Company of
New York and to hold any investments in bearer form, but the books and records
of the Trustee shall at all times show that all such investments are part of
the Trust.

(iv) To keep that portion of the Trust in cash or cash balances as
the Named Fiduciary or Administrator may, from time to time, specifically
instruct the Trustee.

(v) To make, execute, acknowledge, and deliver any and all
documents of transfer or conveyance and to carry out the powas herein granted.

(vi) To settle, compromise, or submit to arbitration any claims,
debts, or damages due to or arising from the Trust; to commence or defend suits
or legal or administrative proceedings; to represent the Trust in all suits and
legal and administrative hearings; and to pay all reasonable expenses arising
from any such action, from the Trust if not paid by the Sponsor.

(vii) With prior written approval from the Sponsor to employ legal,
accounting, clerical, and other assistance as may be required in carrying out
the provisions of this Agreement and to pay their reasonable expenses and
compensation from the Trust if not paid by the Sponsor.

(viii) To do all other acts although not specifically mentioned
herein, as the Trustee may deem necessary to carry out any of the foregoing
powers and the purposes of the Trust.

SECTION 5. RECORDKEEPING AND ADMINISTRATIVE SERVICES TO BE PERFORMED.

(a) GENERAL. The Trustee shall perform those recordkeeping and
administrative functions described in Schedule "A" attached hereto. These
recordkeeping and administrative functions shall be performed within the
framework of the Administrator's written directions regarding the Plan's
provisions, guidelines and interpretations.

(b) ACCOUNTS. The Trustee shall keep accurate accounts of all
investments, receipts, disbursements, and other transactions hereunder, and
shall report the value of the assets held in the Trust as of the last day of
each fiscal quarter of the Plan and, if not on the last day of a fiscal
quarter, the date on which the Trustee resigns or is removed as provided in
Section 8 of this Agreement or is terminated as provided in Section 10 (a
"Reporting Date"). Within thirty (30) days following each Reporting Date or
within sixty (60) days in the case of a Reporting Date caused by the
resignation or removal of the Trustee, or the termination of this Agreement,
the

Trustee shall file with the Administrator a written account setting forth all
investments, receipts, disbursements, and other transactions effected by the
Trustee between the Reporting Date and the prior Reporting Date, and setting
forth the value of the Trust as of the Reporting Date. Except as otherwise
required under ERISA, upon the expiration of six (6) months from the date of
filing such account with the Administrator, the Trustee shall have no liability
or further accountability to anyone with respect to the propriety of its acts
or transactions shown in such account, except with respect to such acts or
transactions as to which the Sponsor shall within such six (6) month period
file with the Trustee written objections.

(c) INSPECTION AND AUDIT. All records generated by the Trustee in
accordance with paragraphs (a) and (b) shall be open to inspection and audit,
during the Trustee's regular business hours prior to the termination of this
Agreement, by the Administrator or any person designated by the Administrator.
Upon the resignation or removal of the Trustee or the termination of this
Agreement, the Trustee shall provide to the Administrator, at no expense to the
Sponsor, in the format regularly provided to the Administrator, a statement of
each participant's accounts including trial balance and/or normal
administrative reports as of the resignation, removal, or termination, and the
Trustee shall provide to the Administrator or the Plan's new recordkeeper such
further records as are reasonable, at the Sponsor's expense.

(d) EFFECT OF PLAN AMENDMENT. A confirmation of the current qualified
status of the Plan is attached hereto as Schedule FN. The Trustee's provision
of the recordkeeping and administrative services set forth in this Section 5
shall be conditioned on the Sponsor delivering to the Trustee a copy of any
amendment to the Plan as soon as administratively feasible following the
amendment's adoption, with, if requested, an IRS determination letter or an
opinion of counsel substantially in the form of Schedule NF covering such
amendment, and on the Administrator providing the Trustee on a timely basis
with all the information the Administrator deems necessary for the Trustee to
perform the recordkeeping and administrative services and such other
information as the Trustee may reasonably request.

(e) RETURNS, REPORTS AND INFORMATION. The Administrator shall be
responsible for the preparation and filing of all returns, reports, and
information required of the Trust or Plan by law. The Trustee shall provide the
Administrator with such information as the Administrator may reasonably request
to make these filings. Unless otherwise agreed upon by the Sponsor and

the Trustee, the Administrator shall also be responsible for making any
disclosures to Participants required by law except, such disclosures as may be
required under federal or state truth-in-lending laws with regard to
Participant loans which shall be provided by the Trustee.

SECTION 6. COMPENSATION AND EXPENSES. Within thirty (30) days of receipt of the
Trustee's ▇▇▇▇, which shall be computed and billed in accordance with Schedule
"B" attached hereto and made a part hereof, as amended from time to time, the
Sponsor shall send to the Trustee a payment in such amount or the Sponsor may
direct the Trustee to deduct such amount from participants' accounts. All
expenses of the Trustee relating directly to the acquisition and disposition of
investments constituting part of the Trust, and all taxes of any kind
whatsoever that may be levied or assessed under existing or future laws upon or
in respect of the Trust or the income thereof, shall be a charge against and
paid from the appropriate Plan participants' accounts.

SECTION 7. DIRECTIONS AND INDEMNIFICATION.

(a) IDENTITY OF ADMINISTRATOR AND NAMED FIDUCIARY. The Trustee shall
be fully protected in relying on the fact that the Named Fiduciary and the
Administrator under the Plan are the individuals or persons named as such above
or such other individuals or persons as the Sponsor may notify the Trustee in
writing.

(b) DIRECTIONS FROM ADMINISTRATOR. Whenever the Administrator provides
a direction to the Trustee, the Trustee shall not be liable for any loss, or by
reason of any breach, arising from the direction if the direction is contained
in a writing (or is oral and immediately confirmed in a writing) signed by any
individual whose name and signature have been submitted (and not withdrawn) in
writing to the Trustee by the Administrator in the form attached hereto as
Schedule "D", provided the Trustee reasonably believes the signature of the
individual to be genuine, unless it is clear on the direction's face that the
actions to be taken under the direction would be prohibited by the fiduciary
duty rules of ERISA or would be contrary to the terms of the plan or this
Agreement or would otherwise be improper. Such direction may also be made via
EDT in accordance with procedures agreed to by the Administrator and the
Trustee; provided, however, that the Trustee shall be fully protected in
relying on such direction as if it were a direction made in writing by the
Administrator. The Trustee shall have no responsibility to ascertain any
direction's (i) accuracy, (ii) compliance with the terms of the Plan or any
applicable law, or (iii) effect for tax purposes or otherwise.

(c) DIRECTIONS FROM NAMED FIDUCIARY. Whenever the Named Fiduciary or
Sponsor provides a direction to the Trustee, the Trustee shall not be liable
for any loss, or by reason of any breach, arising from the direction (i) if the
direction is contained in a writing (or is oral and immediately confirmed in a
writing) signed by any individual whose name and signature have been submitted
(and not withdrawn) in writing to the Trustee by the Named Fiduciary in the
form attached hereto as Schedule "E" and (ii) if the Trustee reasonably
believes the signature of the individual to be genuine, unless it is clear on
the direction's face that the actions to be taken under the direction would be
prohibited by the fiduciary duty rules of ERISA or would be contrary to the
terms of the Plan or this Agreement or would otherwise be improper.

(d) CO-FIDUCIARY LIABILITY. In any other case, the Trustee shall not
be liable for any loss, or by reason of any breach, arising from any act or
omission of another fiduciary under the Plan except as provided in section
405(a) of ERISA.

(e) INDEMNIFICATION. The Sponsor shall indemnify the Trustee against,
and hold the Trustee harmless from, any and all loss, damage, penalty,
liability, cost, and expense, including without limitation, reasonable
attorneys' fees and disbursements, that may be incurred by or, imposed upon the
Trustee by reason of any claim, regulatory proceeding, or litigation arising
from any act done or omitted to be done by any individual or person with
respect to the Plan or Trust, excepting only any and all loss, etc., arising
from the mistake, negligence or bad faith of the Trustee (or any agent
affiliate or contractor of the Trustee performing the Trustee's duties
hereunder) or breach by the Trustee of the terms of this Agreement.

(f) SURVIVAL. The provisions of this Section 7 shall survive the
termination of this Agreement.

SECTION 8. RESIGNATION OR REMOVAL OF TRUSTEE.

(a) RESIGNATION. The Trustee may resign at any time upon sixty (60)
days' notice in writing to the Sponsor, unless a shorter period of notice is
agreed upon by the Sponsor.

(b) REMOVAL. The Sponsor may remove the Trustee at any time upon sixty
(60) days' notice in writing to the Trustee, unless a shorter period of notice
is agreed upon by the Trustee.

SECTION 9. SUCCESSOR TRUSTEE.

(a) APPOINTMENT. If the office of Trustee becomes vacant for any
reason, the Sponsor may in writing appoint a successor trustee under this
Agreement. The successor trustee shall have all of the rights, powers,
privileges, obligations, duties, liabilities, and immunities granted to the
Trustee under this Agreement. The successor trustee and predecessor trustee
shall not be liable for the acts or omissions of the other with respect to the
Trust.

(b) ACCEPTANCE. When the successor trustee accepts its appointment
under this Agreement in writing, title to and possession of the Trust assets
shall immediately vest in the successor trustee without any further action on
the part of the predecessor trustee. The predecessor trustee shall execute all
instruments and do all acts that reasonably may be necessary or reasonably may
be requested in writing by the Sponsor or the successor trustee to vest title
to all Trust assets in the successor trustee or to deliver all Trust assets to
the successor trustee.

(c) RECORDKEEPING. If as a result of the appointment of a successor
trustee hereunder, the Trustee is no longer performing recordlceeping and
administrative services under Section S hereof, the Trustee will furnish copies
of plan records to the Sponsor or to a successor recordkeeper designated by the
Sponsor, and will cooperate as reasonably requested by the Sponsor in effecting
the transition of recordkeeping functions to the successor recordkeeper.

(d) CORPORATE ACTION. Any successor of the Trustee or successor
trustee, through sale or transfer of the business or trust department of the
Trustee or successor trustee, or through reorganization, consolidation, or
merger, or any similar transaction, shall, upon consummation of the
transaction, become the successor trustee under this Agreement.

SECTION 10. TERMINATION. This Agreement may be terminated at any time by the
Sponsor upon sixty (60) days' notice in writing to the Trustee. On the date of
the termination of this

Agreement, the Trustee shall forthwith transfer and deliver to such individuals
or entities as the Sponsor shall designate, all cash and assets then
constituting the Trust and copies of all records and data relating to the
Trustee's recordkeeping and administrative services under Section S hereof. If,
by the termination date, the Sponsor has not notified the Trustee in writing as
to whom the assets and cash are to be transferred and delivered, the Trustee
may bring an appropriate action or proceeding for leave to deposit the assets
and cash in a court of competent jurisdiction. The Trustee shall be reimbursed
by the Sponsor for all costs and expenses of the action or proceeding
including, without limitation, reasonable attorneys' fees and disbursements.

SECTION 11. RESIGNATION, REMOVAL, AND TERMINATION NOTICES. All notices of
resignation, removal, or termination under this Agreement must be in writing
and mailed to the party to which the notice is being given by certified or
registered mail, return receipt requested, or delivered via a receipted
delivery method, to the Sponsor c/o Vice President-General Counsel, Biogen,
Inc., Fourteen ▇▇▇▇▇▇▇▇▇ ▇▇▇▇▇▇, ▇▇▇▇▇▇▇▇▇, ▇▇ ▇▇▇▇▇, and to the Trustee c/o
▇▇▇▇ ▇. ▇▇▇▇▇▇, Fidelity Investments, ▇▇ ▇▇▇▇▇▇▇▇▇▇ ▇▇▇▇▇▇, ▇▇▇▇▇▇,
▇▇▇▇▇▇▇▇▇▇▇▇▇ ▇▇▇▇▇, or to such other individuals addresses as the parties have
notified each other of in the foregoing manner.

SECTION 12. DURATION. This Trust shall continue in effect without limit
as to time, subject, however, to the provisions of this Agreement
relating to amendment, modification, and termination thereof.

SECTION 13. AMENDMENT OR MODIFICATION. This Agreement may be amended or
modified at any time and from time to time only by an instrument executed by
both the Sponsor and the Trustee. Notwithstanding the foregoing, to reflect
increased operating costs the Trustee may once each calendar year (beginning
April 1, 1996) amend Schedule "B" without the Sponsor's consent upon
seventy-five (75) days advance written notice to the Sponsor.

SECTION 14. GENERAL.

(a) PERFORMANCE BY TRUSTEE. ITS AGENTS OR AFFILIATES. The Sponsor
acknowledges and authorizes that the services to be provided under this
Agreement shall be provided by the Trustee, its agents or afffiliates,
including Fidelity Investments Institutional Operations

Company or its successor, and that certain of such services may be provided
pursuant to one or more other contractual agreements or relationships.

(b) ENTIRE AGREEMENT. This Agreement contains all of the terms
agreed upon between the parties with respect to the subject matter hereof.

(c) WAIVER. No waiver by either party of any failure or refusal to
comply with an obligation hereunder shall be deemed a waiver of any other or
subsequent failure or refusal to so comply.

(d) SUCCESSORS AND ASSIGNS. The stipulations in this Agreement shall
inure to the benefit of, and shall bind, the successors and assigns of the
respective parties.

(e) PARTIAL INVALIDITY. If any term or provision of this Agreement or
the application thereof to any person or circumstances shall, to any extent, be
invalid or unenforceable, the remainder of this Agreement, or the application
of such term or provision to persons or circumstances other than those as to
which it is held invalid or unenforceable, shall not be affected thereby, and
each term and provision of this Agreement shall be valid and enforceable to the
fullest extent permitted by law.

(f) SECTION HEADINGS. The headings of the various sections and
subsections of this Agreement have been inserted only for the purposes of
convenience and are not part of this Agreement and shall not be deemed in any
mannerto modify, explain, expand or restrict any of the provisions of this
Agreement.

SECTION 15. GOVERNING LAW.

(a) MASSACHUSETTS LAW CONTROLS. This Agreement is being made in the
Commonwealth of Massachusetts, and the Trust shall be administered as a
Massachusetts trust. The validity, construction, effect, and administration of
this Agreement shall be governed by and interpreted in accordance with the laws
of the Commonwealth of Massachusetts, except to the extent those laws are
superseded under section 514 of ERISA.

(b) TRUST AGREEMENT CONTROLS. The Trustee is not a party to the
Plan, and in the event of any conflict between the provisions of the
Plan and the provisions of this Agreement, the provisions of this
Agreement shall control.

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed by their duly authorized officers as of the day and year first above
written.

BIOGEN, INC.

Attest: /s/ ▇▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇ By: /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇.
---------------------------- -----------------------------------
Secretary ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇.
Vice President-Human Resources

FIDELITY MANAGEMENT TRUST COMPANY

Attest: /s/ By: /s/
---------------------------- -----------------------------------
Clerk Senior Vice President

SCHEDULE "A"
RECORDKEEPING & ADMINISTRATIVE SERVICES

ADMINISTRATION

* Establishment and maintenance of participant account and election
percentages.

* Maintenance of six (6) plan investment options:
-Fidelity Retirement Government Money Market Portfolio
-Fidelity Intermediate Bond Portfolio
-Fidelity Puritan Fund
-Fidelity Magellan Fund
-Fidelity Growth Company Fund
-Biogen Stock - (Employer matching contributions/accounts only)

* All five Fidelity Mutual Funds shall be eligible for the following money
classifications other than Employer Match.

* Maintenance of four (4) money classifications:
-Basic Account
-Supplemental Account
-Rollover
-Employer Match

* Processing of mutual fund trades.

* The Trustee will provide only the recordkeeping and administrative services
set forth on this Schedule "A" and no others.

PROCESSING
* Bi-weekly processing of cnntribution data.
* Daily processing of transfers and changes of future allocations.
* Bi-weekly processing of loans, withdrawals, and distributions.

OTHER SERVICES AS MORE FULLY DETERMINED IN THE OCTOBER 6, 1993 LETTER OF
UNDERSTANDING
* Processing of corrective distributions for 401(k)/401(m) compliance,
including calculation of earnings.
* Monthly trial balance
* Quarterly administrative reports
* Quarterly participant statements
* 1099-Rs
* Participant Loans
* Performance of section 401 (k)/401 (m) limitation testing upon request. In
order to obtain this service, the client shall be required to provide the
information identified in the Fidelity Discrimination Testing Package
Guidelines.
* Employee communications describing available investment options, including
multimedia informational materials and group presentations.
* In-service and hardship withdrawals as approved and directed by the Sponsor.

23
24

BIOGEN, INC. FIDELITY MANAGEMENT TRUST COMPANY

By: /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 4/1/94 By: /s/ 4/14/97
--------------------------------- ------------------------------------
Date Date

24
25

SCHEDULE "B"
FEE SCHEDULE

Annual Recordkeeping Fee $80,000 per year*, billed and payable
quarterly.

Loan Fee Establishment fee of S35.00 per loan
account; annual fee of $15.00 per loan
account.

Return of Excess Fee $25.00 per participant, a one-time charge
per calculation and check generation.

Remote Access Fee $1,000 per year, plus a monthly charge for
TYMNET usage. A one-time installation fee of
$1,500 will also be charged to the sponsor
in the first year.

Other Fees: ADP testing, extraordinary expenses resulting from large numbers of
simultaneous manual transactions or from errors not caused by Fidelity, or for
reports not contemplated in this Agreement. The Administrator may withdraw
reasonable administrative fees from the Trust by written direction to the
Trustee.

* This fee will be imposed pro rata for each CALENDAR quarter during the year.
If the Sponsor terminates this Agreement before the end of a calendar yeadr then
the recordkeeping fee will be pro-rated only for the period of time prior to
termination. There will be no recordkeeping fee charged after termination of the
Agreement.

TRUSTEE FEES:

To the extent that assets are invested in Sponsor Stock, .25% of such assets in
the Trust payable pro rata quarterly on the basis of such assets as of the
calendar quarter's last valuation date, subject to a $10,000 annual minimum.

Note: These fees have been negotiated and accepeed based on current plan assets
of $6.0 million, current participation of 300 participants and projected net
cash flows of $.9 million per year. Fees will be subject to revision if these
Plan tharacteristics change significantly by either falling below or exceeding
current projected levels. Fees also have been based on the use of up to six (6)
Fidelity managed investment options, and such fees will be subject to revision
if additional investment options are added.

Increases in participant levels have been contemplated and would not impact fees
until after the 2 year fee guarantee.

BIOGEN, INC. FIDELITY MANAGEMENT TRUST COMPANY

BY: /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 4/1/94 BY: /s/ 4/14/94
--------------------------------- -----------------------------------
Date Senior Vice President Date

SCHEDULE "C"

INVESTMENT OPTIONS

In accordance with Section 4(b), the Named Fiduciary hereby directs the
Trustee that participants' individual accounts may be invested in the following
investment options:

-Fidelity Retirement Government Money Market Portfolio
-Fidelity Intermediate Bond Portfolio
-Fidelity Puritan Fund
-Fidelity Magellan Fund
-Fidelity Growth Company Fund
-Biogen Stock - (Employer matching contributions/accounts only)

The mutual fund advised by Fidelity Management & Research Company referred
to in Section 4(c) shall be Fidelity Retirement Government Money Market
Portfolio.

BIOGEN, INC.

BY: /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 4/1/94
---------------------------------
DATE

SCHEDULE "G"

TELEPHONE EXCHANGE PROCEDURES

The following telephone exchange procedures are currently employed by Fidelity
Institutional Retirement Services Company (FIRSCO).

Telephone exchange hours are 8:30 a.m. (EST) to 8:00 p.m. (EST) on each
business day. A "business day" is any day on which the New York Stock Exchange
is open.

FIRSCO reserves the right to change these telephone exchange procedures at its
discretion and agrees to communicate them immediately to Sponsor.

MUTUAL FUNDS

EXCHANGES BETWEEN MUTUAL FUNDS

Participants may call on any business day to exchange between the
mutual funds. If the request is received before 4:00 p.m. (EST), it
will receive that day's trade date. Calls received after 4:00 p.m.
(EST) will be processed on a next business day basis.

BIOGEN, INC.

By: /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 4/1/94
-------------------------------------
Date

FIRST AMENDMENT TO TRUST AGREEMENT BETWEEN
FIDELITY MANAGEMENT TRUST COMPANY AND
BIOGEN, INC

THIS FIRST AMENDMENT, dated as of the first day of January, 1995, by and
between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc. (the
"Sponsor"); and

WITNESSETH:

WHEREAS, the Trustee and the Sponsor heretofore entered into a trust
agreement dated April 1, 1994 with regard to the Biogen Savings Plan (the
"Plan"); and

WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

(1) Amending and adding the following mutual funds to the "investment option"
section to Schedules "A" and "C", as follows:

Fidelity Asset Manager: Growth
Fidelity International Growth & Income Fund

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this First
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC FIDELITY MANAGEMENT TRUST COMPANY

By /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 11/30/94 By /s/ 12/12/94
---------------------------------- -------------------------------------
Date Vice President Date

SECOND AMENDMENT TO TRUST AGREEMENT BETWEEN
FIDELITY MANAGEMENT TRUST COMPANY AND
BIOGEN, INC

THIS SECOND AMENDMENT, dated as of the fifteenth day of October, 1995, by
and between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc.
(the "Sponsor"); and

WITNESSETH:

WHEREAS, the Trustee and the Sponsor heretofore entered into a trust
agreement dated April 1, 1994 with regard to the Biogen Savings Plan (the
"Plan"); and

WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

(1) Amending and adding the following mutual funds to the "investment option"
section to Schedules "A" and "C", as follows:

- Fidelity U.S. Equity Index Portfolio
- Fidelity U.S. Bond Index Portfolio

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Second
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC FIDELITY MANAGEMENT TRUST COMPANY

By /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 9/20/95 By /s/ 10/9/95
---------------------------------- -------------------------------------
Date Vice President Date

THIRD AMENDMENT TO TRUST AGREEMENT BETWEEN
FIDELITY MANAGEMENT TRUST COMPANY AND
BIOGEN, INC.

THIS THIRD AMENDMENT, dated as of the third day of October, 1996, by and
between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc. (the
"Sponsor");

WITNESSETH:

WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

WHEREAS, the Trustee and the Sponsor now desire to amend said trust
agreement as provided for in Section 13 thereof;

NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the trust agreement by:

(1) Amending the "investment options" portion of Schedules "A" and "C"
by adding the following:

Fidelity Value Fund

(2) Amending the Annual Participant Fee on Schedule "B" as follows:

EFFECTIVE OCTOBER 1, 1996:
Annual Recordkeeping Fee: $20.00 per participant, billed
and payable quarterly.

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Third
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC FIDELITY MANAGEMENT TRUST COMPANY

By /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 10/3/96 By /s/ 10/23/96
---------------------------------- -------------------------------------
Date Vice President Date

FOURTH AMENDMENT TO TRUST AGREEMENT BETWEEN
FIDELITY MANAGEMENT TRUST COMPANY AND
BIOGEN, INC.

THIS FOURTH AMENDMENT, dated as of the fifteenth day of September, 1997,
by and between Fidelity Management Trust Company (the "Trustee") and Biogen,
Inc. (the "Sponsor");

WITNESSETH:

WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

WHEREAS, the Sponsor now desires, and hereby directs the Trustee, in
accordance with Section 7(c), to liquidate all participant balances held in the
Fidelity International Growth & Income Fund on February 27, 1998, and to invest
the proceeds in the USAA International Fund. The parties hereto agree that the
Trustee shall have no discretionary authority with respect to this sale and
transfer directed by the Sponsor. Any variation from the procedure described
herein may be instituted only at the express written directions of the Sponsor;
and

WHEREAS, the Trustee and the Sponsor now desire to amend said Trust
Agreement as provided for in Section 13 thereof;

NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the Trust Agreement by:

(1) Amending Section 4(b)(i) AVAILABLE INVESTMENT OPTIONS by redefining
"Mutual Fund" as follows:

(i) securities issued by investment companies advised by Fidelity
Management & Research Company ("Fidelity Mutual Funds") and certain
securities issued by registered investment companies not advised by
Fidelity Management & Research Company ("Non-Fidelity Mutual Funds")
(collectively referred to as "Mutual Funds").

(2) Inserting the following sentence before all others in
subsection 4(d):

All transactions involving Non-Fidelity Mutual Funds shall be done in
accordance with the OPERATIONAL GUIDELINES FOR NON-FIDELITY MUTUAL
FUNDS attached hereto as Schedule "H".

(3) Adding a Schedule "H" OPERATIONAL GUIDELINES FOR
NON-FIDELITY MUTUAL FUNDS as attached.

(4) Amending the "investment options" section of Schedules "A" and "C" by
restating as follows:

- Fidelity Low-Priced Stock Fund
- Janus Worldwide Fund
32

- USAA International Fund
- Fidelity International Growth & Income Fund (frozen to new
investments as of February 27, 1998)
- Fidelity Value Fund
- Spartan U.S. Equity Index Fund
- Fidelity U.S. Bond Index Portfolio
- Fidelity Asset Manager: Growth
- Fidelity Retirement Government Money Market Portfolio
- Fidelity Intermediate Bond Fund
- Fidelity Puritan Fund
- Fidelity Magellan Fund
- Fidelity Growth Company Fund
- Biogen Stock

(5) Amending Schedule "B" by adding "Non-Fidelity Mutual Funds" as
follows:

Non-Fidelity Mutual Funds: .25% annual administration fee on all
Non-Fidelity Mutual Fund assets (to
be paid by the Non-Fidelity Mutual
Fund vendor).

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Fourth
Amendment to be executed by their duly authorized officers effective as of the
day and year first above written.

BIOGEN, INC. FIDELITY MANAGEMENT TRUST COMPANY

By /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 9/12/97 By /s/ ▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇▇ 9/23/97
---------------------------------- -------------------------------------
Date Vice President Date

2
33

SCHEDULE "H"

OPERATIONAL GUIDELINES FOR NON-FIDELITY MUTUAL FUNDS

PRICING

By 7:00 p.m. Eastern Time ("ET") each Business Day, the Non-Fidelity Mutual
Fund Vendor (Fund Vendor) will input the following information ("Price
Information") into the Fidelity Participant Recordkeeping System ("FPRS") via
the remote access price screen that Fidelity Investments Institutional
Operations Company, Inc. ("FIIOC"), an affiliate of the Trustee, has provided
to the Fund Vendor: (1) the net asset value for each Fund at the Close of
Trading, (2) the change in each Fund's net asset value from the Close of
Trading on the prior Business Day, and (3) in the case of an income fund or
funds, the daily accrual for interest rate factor ("mil rate"). FIIOC must
receive Price Information each Business Day (a "Business Day" is any day the
New York Stock Exchange is open). If on any Business Day the Fund Vendor does
not provide such Price Information to FIIOC, FIIOC shall pend all associated
transaction activity in the Fidelity Participant Recordkeeping System
("FPRS") until the relevant Price Information is made available by Fund
Vendor.

TRADE ACTIVITY AND WIRE TRANSFERS

By 7:00 a.m. ET each Business Day following Trade Date ("Trade Date PLUS
One"), FIIOC will provide, via facsimile, to the Fund Vendor a consolidated
report of net purchase or net redemption activity that occurred in each of
the Funds up to 4:00 p.m. ET on the prior Business Day. The report will
reflect the dollar amount of assets and shares to be invested or withdrawn
for each Fund. FIIOC will transmit this report to the Fund Vendor each
Business Day, regardless of processing activity. In the event that data
contained in the 7:00 a.m. ET facsimile transmission represents estimated
trade activity, FIIOC shall provide a final facsimile to the Fund Vendor by
no later than 9:00 a.m. ET. Any resulting adjustments shall be processed by
the Fund Vendor at the net asset value for the prior Business Day.

The Fund Vendor shall send via regular mail to FIIOC transaction confirms for
all daily activity in each of the Funds. The Fund Vendor shall also send via
regular mail to FIIOC, by no later than the fifth Business Day following
calendar month close, a monthly statement for each Fund. FIIOC agrees to
notify the Fund Vendor of any balance discrepancies within twenty (20)
Business Days of receipt of the monthly statement.

For purposes of wire transfers, FIIOC shall transmit a daily wire for
aggregate purchase activity and the Fund Vendor shall transmit a daily wire
for aggregate redemption activity, in each case including all activity across
all Funds occurring on the same day.

PROSPECTUS DELIVERY

FIIOC shall be responsible for the timely delivery of Fund prospectuses and
periodic Fund reports ("Required Materials") to Plan participants, and shall
retain the services of a third-party vendor to handle such mailings. The Fund
Vendor shall be responsible for all materials and production costs, and
hereby agrees to provide the Required Materials to the third-party vendor
selected by FIIOC. The Fund Vendor shall bear the costs of mailing annual
Fund reports to Plan participants. FIIOC shall bear the costs of mailing
prospectuses to Plan participants.

PROXIES

The Fund Vendor shall be responsible for all costs associated with the
production of proxy materials. FIIOC shall retain the services of a
third-party vendor to handle proxy solicitation mailings and vote tabulation.
Expenses associated with such services shall be billed directly to the Fund
Vendor by the third-party vendor.

PARTICIPANT COMMUNICATIONS

The Fund Vendor shall provide internally-prepared fund descriptive
information approved by the Funds' legal counsel for use by FIIOC in its
written participant communication materials. FIIOC shall utilize historical
performance data obtained from third-party vendors (currently Morningstar,
Inc., FACTSET Research Systems and Lipper Analytical Services) in telephone
conversations with plan participants and in quarterly participant statements.
The Sponsor hereby consents to FIIOC's use of such materials and acknowledges
that FIIOC is not responsible for the accuracy of such third-party
information. FIIOC shall seek the approval of the Fund Vendor prior to
retaining any other third-party vendor to render such data or materials under
this Agreement.

COMPENSATION

FIIOC shall be entitled to fees as set forth in a separate agreement with the
Fund Vendor.

4
35

FIFTH AMENDMENT TO TRUST AGREEMENT BETWEEN
FIDELITY MANAGEMENT TRUST COMPANY AND
BIOGEN, INC.

THIS FIFTH AMENDMENT, dated as of the fifteenth day of December, 1997, by
and between Fidelity Management Trust Company (the "Trustee") and Biogen, Inc.
(the "Sponsor");

WITNESSETH:

WHEREAS, the Trustee and the Sponsor heretofore entered into a Trust
Agreement dated April 1, 1994, with regard to the Biogen Savings Plan (the
"Plan"); and

WHEREAS, the Trustee and the Sponsor now desire to amend said Trust
Agreement as provided for in Section 13 thereof;

NOW THEREFORE, in consideration of the above premises the Trustee and the
Sponsor hereby amend the Trust Agreement by:

(1) Amending Section 4(e) by restating as follows:

(e) SPONSOR STOCK. Trust investments in Sponsor Stock shall be made
via the Biogen Stock Fund (the "Stock Fund"). Investments in the Stock Fund
shall consist primarily of shares of Sponsor Stock. In order to satisfy daily
participant exchange or withdrawal requests for transfers and payments, the
Stock Fund shall also include cash or short-term liquid investments in
accordance with this paragraph. Such holdings will include Fidelity
Institutional Cash Portfolios: Money Market Portfolio: Class I or such other
Mutual Fund or commingled money market pool as agreed to by the Sponsor and
Trustee. The Named Fiduciary shall, after consultation with the Trustee,
establish and communicate to the Trustee in writing a target percentage and
drift allowance for such short-term liquid investments. The Trustee shall be
responsible for ensuring that the actual cash held in the Stock Fund falls
within the agreed upon range over time. Each participant's proportional interest
in the Stock Fund shall be measured in units of participation, rather than
shares of Sponsor Stock. Such units shall represent a proportionate interest in
all of the assets of the Stock Fund, which includes shares of Sponsor Stock,
short-term investments and at times, receivables for dividends and/or Sponsor
Stock sold and payables for Sponsor Stock purchased. The Trustee shall determine
a daily net asset value ("NAV") for each unit outstanding of the Stock Fund.
Valuation of the Stock Fund shall be based upon the 4:00 p.m. NASDQ closing
price of the stock, or if unavailable, the latest available price as reported by
the principal national securities exchange on which the Sponsor Stock is traded.
The NAV shall be adjusted by dividends paid on the shares of Sponsor Stock held
by the Stock Fund, gains or losses realized on sales of Sponsor Stock,
appreciation or depreciation in the market price of those shares owned, and
interest on the short-term investments held by the Stock Fund, expenses

that, pursuant to Sponsor direction, the Trustee accrues from the Stock Fund,
and commissions on purchases and sales of Sponsor Stock. Investments in the
Stock Fund shall be subject to the following limitations:

(i) ACQUISITION LIMIT. Pursuant to the Plan, the Trust
may be invested in Sponsor Stock to the extent necessary to comply with
investment directions in accordance with this Agreement.

(ii) FIDUCIARY DUTY OF NAMED FIDUCIARY. The Named Fiduciary
shall continually monitor the suitability under the fiduciary duty rules of
section 404(a)(1) of ERISA (as modified by section 404(a)(2) of ERISA) of
acquiring and holding Sponsor Stock. The Trustee shall not be liable for any
loss, or by reason of any breach, which arises from the directions of the Named
Fiduciary with respect to the acquisition and holding of Sponsor Stock, unless
it is clear on their face that the actions to be taken under those directions
would be prohibited by the foregoing fiduciary duty rules or would be contrary
to the terms of this Agreement.

(iii) Purchase and sales of Sponsor Stock shall be made on the
open market as necessary to maintain the target cash percentage and drift
allowance for the Stock Fund, provided that:

(A) If the Trustee is unable to purchase or sell
the total number of shares required to be purchased or sold on such day as
a result of market conditions; or

(B) If the Trustee is prohibited by the Securities
and Exchange Commission, the NASDQ, or any other regulatory body from purchasing
or selling any or all of the shares required to be purchased or sold on such
day, then the Trustee shall purchase or sell such shares as soon as possible
thereafter. The Trustee may follow directions from the Administrator or Named
Fiduciary to deviate from the above purchase and sale procedures provided that
such direction is made in writing by the Administrator or Named Fiduciary.

(iv) EXECUTION OF PURCHASES AND SALES.

(A) Purchases and sales of units in the Stock Fund (other than
for exchanges) shall be made on the date on which the Trustee receives from the
Administrator in good order all information, documentation, and wire transfers
of funds (if applicable), necessary to accurately effect such transactions.
Exchanges of units in the Stock Fund shall be made in accordance with the
Telephone Exchange Guidelines attached hereto as Schedule "G". The Trustee may
follow directions from the Administrator or Named Fiduciary

to deviate from the above purchase and sale procedures provided that such
direction is made in writing by the Administrator or Named Fiduciary.

(B) PURCHASES AND SALES FROM OR TO SPONSOR. If directed by
the Sponsor in writing prior to the trading date, the Trustee may purchase or
sell Sponsor Stock from or to the Sponsor if the purchase or sale is for
adequate consideration (within the meaning of section 3(18) of ERISA) and no
commission is charged. If Sponsor contributions (employer) or contributions made
by the Sponsor on behalf of the participants (employee) under the Plan are to be
invested in Sponsor Stock, the Sponsor may transfer Sponsor Stock in lieu of
cash to the Trust. In either case, the number of shares to be transferred will
be determined by dividing the total amount of Sponsor Stock to be purchased or
sold by the 4:00 p.m. NASDQ closing price of the Sponsor Stock on the trading
date.

(C) USE OF AN AFFILIATED BROKER. The Sponsor hereby
directs the Trustee to use Fidelity Brokerage Services, Inc. ("FBSI") to provide
brokerage services in connection with any purchase or sale of Sponsor Stock
which results from directions received from Plan participants. FBSI shall
execute such directions directly or through its affiliate, National Financial
Services Company ("NFSC"). The provision of brokerage services shall be subject
to the following:

(1) As consideration for such brokerage services, the
Sponsor agrees that FBSI shall be entitled to remuneration under this
authorization provision in the amount of three and one-half cents ($.035)
commission on each share of Sponsor Stock. Any change in such remuneration may
be made only by a signed agreement between Sponsor and Trustee.

(2) Following the procedures set forth in Department
of Labor Prohibited Transaction Class Exemption 86-128 (PTCE 86-128), the
Trustee will provide the Sponsor with the following documents: (1) a description
of FBSI's brokerage placement practices; (2) a copy of PTCE 86-128; and (3) a
form by which the Sponsor may terminate this direction to use a broker
affiliated with the Trustee. The Trustee will provide the Sponsor with this
termination form annually, as well as quarterly and annual reports which
summarize all securities transaction-related charges incurred by the Plan.

(3) Any successor organization of FBSI, through
reorganization, consolidation, merger or similar transactions, shall, upon
consummation of such transaction, become the successor broker in accordance with
the terms of this authorization provision.

(4) The Trustee and FBSI shall continue to rely on
this authorization provision until notified to the contrary. The Sponsor
reserves the right to terminate this authorization upon written notice to FBSI
(or its successor) and the Trustee, in accordance with Section 11 of this
Agreement.

(iv) SECURITIES LAW REPORTS. The Named Fiduciary shall be
responsible for filing all reports required under Federal or state securities
laws with respect to the Trust's ownership of Sponsor Stock, including, without
limitation, any reports required under section 13 or 16 of the Securities
Exchange Act of 1934, and shall immediately notify the Trustee in writing of any
requirement to stop purchases or sales of Sponsor Stock pending the filing of
any report. The Trustee shall provide to the Named Fiduciary such information on
the Trust's ownership of Sponsor Stock as the Named Fiduciary may reasonably
request in order to comply with Federal or state securities laws.

(v) VOTING AND TENDER OFFERS. Notwithstanding any other
provision of this Agreement the provisions of this Section shall govern the
voting and tendering of Sponsor Stock. The Sponsor, after consultation with the
Trustee, shall provide and pay for all printing, mailing, tabulation and other
costs associated with the voting and tendering of Sponsor Stock.

(A) VOTING.

(1) When the issuer of Sponsor Stock prepares for any
annual or special meeting, the Sponsor shall notify the Trustee at least thirty
(30) days in advance of the intended record date and shall cause a copy of all
proxy solicitation materials to be sent to the Trustee. If requested by the
Trustee, the Sponsor shall certify to the Trustee that the aforementioned
materials represents the same information that is distributed to shareholders of
Sponsor Stock. Based on these materials the Trustee shall prepare a voting
instruction form and shall provide a copy of all proxy solicitation materials to
be sent to each Plan participant with an interest in Sponsor Stock held in the
Trust, together with the foregoing voting instruction form to be returned to the
Trustee or its designee. The form shall show the proportional interest in the
number of full and fractional shares of Sponsor Stock credited to the
participant's accounts held in the Stock Fund.

(2) Each participant with an interest in the Stock
Fund shall have the right to direct the Trustee as to the manner in which the
Trustee is to vote (including not to vote) that number of shares of Sponsor
Stock reflecting such participant's proportional interest in the Stock Fund
(both vested and

unvested). Directions from a participant to the Trustee concerning the voting of
Sponsor Stock shall be communicated in writing, or by mailgram or similar means
as is agreed upon by the Trustee and the Sponsor. These directions shall be held
in confidence by the Trustee and shall not be divulged to the Sponsor, or any
officer or employee thereof, or any other person except to the extent that the
consequences of such directions are reflected in reports regularly communicated
to any such persons in the ordinary course of the performance of the Trustee's
services hereunder. Upon its receipt of the directions, the Trustee shall vote
the shares of Sponsor Stock reflecting the participant's proportional interest
in the Stock Fund as directed by the participant. Except as otherwise required
by law, the Trustee shall not vote shares of Sponsor Stock reflecting a
participant's proportional interest in the Stock Fund for which it has received
no direction from the participant.

(3) The Trustee shall vote that number of shares of
Sponsor Stock not credited to participants' accounts in the same proportion on
each issue as it votes those shares credited to participants' accounts for which
it received voting directions from participants.

(B) TENDER OFFERS.

(1) Upon commencement of a tender offer for any
securities held in the Trust that are Sponsor Stock, the Sponsor shall timely
notify the Trustee in advance of the intended tender date and shall cause a copy
of all materials to be sent to the Trustee. The Sponsor shall certify to the
Trustee that the aforementioned materials represent the same information
distributed to shareholders of Sponsor Stock. Based on these materials and after
consultation with the Sponsor the Trustee shall prepare a tender instruction
form and shall provide a copy of all tender materials to be sent to each plan
participant, together with the foregoing tender instruction form, to be returned
to the Trustee or its designee. The tender instruction form shall show the
number of full and fractional shares of Sponsor Stock that reflect the
participants proportional interest in the Stock Fund (both vested and unvested).

(2) Each participant shall have the right to direct
the Trustee to tender or not to tender some or all of the shares of Sponsor
Stock reflecting such participant's proportional interest in the Stock Fund
(both vested and unvested). Directions from a participant to the Trustee
concerning the tender of Sponsor Stock shall be communicated in writing, or by
mailgram or such similar means as is agreed upon by the Trustee and the Sponsor.
These directions shall be held in confidence by the Trustee and shall not be
divulged to the Sponsor, or any officer or employee thereof, or any other person
except to the extent that the consequences of such directions are reflected in
reports regularly communicated to any such persons in the ordinary course of the

performance of the Trustee's services hereunder. The Trustee shall tender or not
tender shares of Sponsor Stock as directed by the participant. Except as
otherwise required by law, the Trustee shall not tender shares of Sponsor Stock
reflecting a participant's proportional interest in the Stock Fund for which it
has received no direction from the participant.

(3) Except as otherwise required by law, the Trustee
shall tender that number of shares of Sponsor Stock not credited to
participants' accounts in the same proportion as the total number of shares of
Sponsor Stock credited to participants' accounts for which it has received
instructions from Participants.

(4) A participant who has directed the Trustee to
tender some or all of the shares of Sponsor Stock reflecting the participant's
proportional interest in the Stock Fund may, at any time prior to the tender
offer withdrawal date, direct the Trustee to withdraw some or all of the
tendered shares reflecting the participant's proportional interest, and the
Trustee shall withdraw the directed number of shares from the tender offer prior
to the tender offer withdrawal deadline. Prior to the withdrawal deadline, if
any shares of Sponsor Stock not credited to participants' accounts have been
tendered, the Trustee shall redetermine the number of shares of Sponsor Stock
that would be tendered under Section 4(e)(v)(B)(3) if the date of the foregoing
withdrawal were the date of determination, and withdraw from the tender offer
the number of shares of Sponsor Stock not credited to participants' accounts
necessary to reduce the amount of tendered Sponsor Stock not credited to
participants' accounts to the amount so redetermined. A participant shall not be
limited as to the number of directions to tender or withdraw that the
participant may give to the Trustee.

(5) A direction by a participant to the Trustee to
tender shares of Sponsor Stock reflecting the participant's proportional
interest in the Stock Fund shall not be considered a written election under the
Plan by the participant to withdraw, or have distributed, any or all of his
withdrawable shares. The Trustee shall credit to each proportional interest of
the participant from which the tendered shares were taken the proceeds received
by the Trustee in exchange for the shares of Sponsor Stock tendered from that
interest. Pending receipt of directions (through the Administrator) from the
participant or the Named Fiduciary, as provided in the Plan, as to which of the
remaining investment options the proceeds should be invested in, the Trustee
shall invest the proceeds in the investment option described in Schedule "C".

(vi) GENERAL. With respect to all rights other than the right
to vote, the right to tender, and the right to withdraw shares previously
tendered, in the case of Sponsor Stock credited to a

participant's proportional interest in the Stock Fund, the Trustee shall follow
the directions of the participant and if no such directions are received, the
directions of the Named Fiduciary. The Trustee shall have no duty to solicit
directions from participants. With respect to all rights other than the right to
vote and the right to tender, in the case of Sponsor Stock not credited to
participants' accounts, the Trustee shall follow the directions of the Named
Fiduciary.

(vii) CONVERSION. All provisions in this Section 4(e) shall
also apply to any securities received as a result of a conversion of Sponsor
Stock.

(2) Amending Section 4(h), TRUSTEE POWERS, by adding the following:

(ix) To borrow funds from a bank not affiliated with the Trustee in
order to provide sufficient liquidity to process Plan transactions in
a timely fashion; provided that the cost of such borrowing shall be
allocated in a reasonable fashion to the investment fund(s) in need
of liquidity.

(3) Amending the "investment options" section of Schedules "A" and "C" by
restating as follows:

* Maintenance of the following investment options:

- Fidelity Money Market Trust: Retirement
- Money Market Portfolio
- Fidelity Intermediate Bond Fund
- Fidelity Puritan Fund
- Fidelity Magellan Fund
- Fidelity Growth Company Fund
- Biogen Stock Fund
- Fidelity Asset Manager: Growth
- Spartan U.S. Equity Index Fund
- Fidelity U.S. Bond Index Fund
- Fidelity Value Fund
- Fidelity Low-Priced Stock Fund
- Janus Worldwide Fund
- USAA International Fund

(4) Amending the "money classifications" Section of Schedule "A" by
restating as follows:

* Maintenance of the following money classifications:

- Basic Account
- Supplemental Account
- Rollover
- Employer Match

(5) Restating Schedule "G" TELEPHONE EXCHANGE PROCEDURES as attached.

IN WITNESS WHEREOF, the Trustee and the Sponsor have caused this Fifth Amendment
to be executed by their duly authorized officers effective as of the day and
year first above written.

BIOGEN, INC. FIDELITY MANAGEMENT TRUST COMPANY

By /s/ ▇▇▇▇▇ ▇. ▇▇▇▇▇, ▇▇. 11/4/97 By /s/ 12/1/97
----------------------------------- -----------------------------------
Date Vice President Date

8
43

SCHEDULE "G"

TELEPHONE EXCHANGE GUIDELINES

The following telephone exchange guidelines are currently employed by Fidelity
Institutional Retirement Services Company (FIRSCO).

Telephone exchange hours are 8:30 a.m. (ET) to 8:00 p.m. (ET) on each business
day. A "business day" is any day on which the New York Stock Exchange is open.

FIRSCO reserves the right to change these telephone exchange guidelines at its
discretion.

MUTUAL FUNDS

EXCHANGES BETWEEN MUTUAL FUNDS

Participants may call on any business day to exchange between the mutual funds.
If the request is received before 4:00 p.m. (ET), it will receive that day's
trade date. Calls received after 4:00 p.m. (ET) will be processed on a next
business day basis.

SPONSOR STOCK

I. EXCHANGES FROM MUTUAL FUNDS INTO SPONSOR STOCK

Sponsor Stock exchanges are processed on a monthly cycle. Participants who
wish to exchange out of a mutual fund into Sponsor Stock may call between
the 1st and the 15th of the month. No calls will be accepted after 4:00
p.m. (ET) on the 15th (or previous business day if the 15th is not a
business day).

Mutual fund shares are sold on the 15th of the month (or the previous
business day if the 15th is not a business day) and the Sponsor Stock is
purchased within two (2) business days after the date on which the mutual
fund shares are sold.

II. EXCHANGES FROM SPONSOR STOCK INTO MUTUAL FUNDS

Participants who wish to exchange out of Sponsor Stock into mutual funds
may call between the 1st and the 15th of the month. No calls will be
accepted after 4:00 p.m. (ET) on the 15th (or previous business day if the
15th is not a business day).

The Sponsor Stock is sold on the 16th (or the next business day if the
16th is not a business day) and the subsequent purchase into mutual funds
will take place three (3) business days later. This allows for settlement
of the stock trade at the custodian and the corresponding transfer to
Fidelity.

BIOGEN, INC.

By: __________________________
Date

TRUST AGREEMENT Between BIOGEN, INC. And

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