July 12, 2004 Dr. Ram Sasisekharan

Exhibit 10.1

July 12, 2004

Dr. Xxx Xxxxxxxxxxxx

0000 Xxxxxxxxxxxxx Xxxxxx, 0X

Xxxxxxxxx, Xxxxxxxxxxxxx 00000

Dear Ram:

Reference is made to the Consulting Agreement between Momenta Pharmaceuticals, Inc. (formerly Mimeon, Inc.) and you dated August 16, 2001, as amended by letter agreement dated August 1, 2003 (the “Agreement”).  Capitalized terms used herein and not otherwise defined shall have the meanings given such terms in the Agreement.

Pursuant to Section 1 of the Agreement, Company and Consultant hereby agree to extend the term of the Agreement for one additional year, from August 16, 2004 through August 15, 2005, upon the same terms and conditions, including, without limitation, those governing compensation, as are set forth in the Agreement.

Consultant hereby represents that Consultant has not been debarred, and to the best of Consultant’s knowledge, is not under consideration to be debarred, by the Food and Drug Administration from working in or providing services to any pharmaceutical or biotechnology company under the Generic Drug Enforcement Act of 1992.

If the foregoing is in conformity with your understanding, please sign both copies of this letter agreement and return one fully-executed copy to me.

Very truly yours,

/s/ Xxxx X. Xxxxx

Xxxx X. Xxxxx

Chief Executive Officer

Agreed and accepted:

/s/ Xxx Xxxxxxxxxxxx

Xxx Xxxxxxxxxxxx