April 30, 1996 Acusphere, Inc. 38 Sidney Street Cambridge, MA 02139 Dear Ladies and Gentlemen: For good and valuable consideration, the receipt and sufficiency of which is hereby acknowledged, each of the undersigned stockholders of Acusphere, Inc.,...

                                                                   Exhibit 10.06

                                        April 30, 1996


Acusphere, Inc.
00 Xxxxxx Xxxxxx
Xxxxxxxxx, XX 00000

Dear Ladies and Gentlemen:

     For good and valuable consideration, the receipt and sufficiency of which
is hereby acknowledged, each of the undersigned stockholders of Acusphere, Inc.,
(the "Company") hereby agrees that in the event of an underwritten public
offering of the Company's securities, such stockholder will be bound by Section
2.14 of the Amended and Restated Investors' Rights Agreement, dated March 30,
1995 as if such stockholder were an "Investor" under such Agreement.

                                        Very truly yours,


                                        /s/ Xxxxxx X. Xxxxx
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                                        Xxxxxx X. Xxxxx


                                        /s/ Xxxxxx X. Xxxxxx
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                                        Xxxxxx X. Xxxxxx


                                        /s/ Xxxxx X. Xxxxxxx
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                                        Xxxxx X. Xxxxxxx