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                                                                   EXHIBIT 10.26


                               EXCHANGE AGREEMENT

         This Exchange Agreement is made as of the 30th of June, 2000 (the
"Effective Date") by and among Acusphere, Inc., a Delaware corporation
("Acusphere"), and Elan International Services, Ltd., a Bermuda exempted limited
liability company ("Elan").

         WHEREAS, Acusphere and Elan are parties to the Securities Purchase
Agreement dated as of the date hereof (the "Securities Purchase Agreement")
providing, INTER ALIA, for the purchase by Elan of shares of Acusphere's Series
G Non-Voting Convertible Preferred Stock, $.0l par value per share (the "Series
G Preferred Stock"), and Series H Convertible Preferred Stock, $.0l par value
per share (the "Series H Preferred Stock"), and for the purchase by Elan of a
convertible promissory note in the aggregate principal amount of up to
$8,010,000 (the "Notes"), which Notes are convertible, at the option of the
holder, into shares of Acusphere's Common Stock, $.01 par value (all of such
shares into which such Notes are convertible being referred to herein as the
"Note Conversion Shares"); and

         WHEREAS, among the conditions to the consummation of the transactions
contemplated by the Securities Purchase Agreement is the execution and delivery
of an agreement providing for certain rights of exchange with respect to the
Series G Preferred Stock and for certain rights of conversion with respect to
the Series G Preferred Stock, Series H Preferred Stock and the Note Conversion
Shares.

         NOW, THEREFORE, in consideration of mutual promises and covenants set
forth herein, and for other good and valuable consideration, the parties hereto
agree as follows:

1.       EXCHANGE RIGHTS.

         1.1      At any time after the Effective Date, Elan shall have the
right (the "Exchange Right") to exchange all of the then outstanding shares of
Series G Preferred Stock held by it for all of the then outstanding shares of
non-voting convertible preference shares (the "Newco Preferred Stock") of
Acusphere Newco Ltd., a Bermuda exempted limited liability company ("Newco"),
then held by Acusphere; PROVIDED, HOWEVER, that in the event Acusphere has, as
of the Exchange Date (as defined below), converted all of its Newco Preferred
Stock into shares of voting common shares of Newco (the "Newco Common Stock"),
Elan shall have the right to receive 50% of the then outstanding shares of Newco
Common Stock then held by Acusphere; FURTHER PROVIDED, HOWEVER, that the
aggregate number of shares of Newco Preferred Stock or Newco Common Stock, as
the case may be, to be exchanged pursuant to this Section 1 shall be reduced by
an amount equal to 5% of the total number of outstanding shares of capital stock
of Newco (including, without limitation, the Newco Preferred Stock and the Newco
Common Stock) as of the Exchange Date, rounded to the nearest whole number of
shares, for each year that this Exchange Right is not exercised, beginning with
the year commencing on June 30, 2004.

         1.2      In order to exercise this Exchange Right, Elan shall provide
written notice thereof to Acusphere (the "Exchange Notice"), setting forth (i)
the fact that Elan intends to exercise the Exchange Right, and (ii) the proposed
date for such exercise (the "Exchange Date"), which date




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shall be no less than 10 business days and no more than 30 business days after
the date such notice is first delivered to Acusphere. On the Exchange Date, (x)
Elan shall surrender for cancellation a certificate or certificates for all of
its shares of Series G Preferred Stock or Series G Conversion Shares (as defined
below), as the case may be, at the offices of Acusphere or of any transfer agent
of Acusphere and (y) Acusphere shall, as soon as practicable thereafter, cause
to be transferred and delivered to Elan such number of shares of Newco capital
stock as is required to be exchanged pursuant to Section 1.1 hereto. Such
exchange shall be deemed to have been made immediately prior to the close of
business on the Exchange Date, at which time all of the Series G Preferred Stock
so exchanged shall be cancelled. For the avoidance of doubt, any paid-in-kind
dividends due or payable to the holders of the Series G Preferred Stock upon the
Exchange Date shall be deemed canceled.

         1.3      In the event that the Series G Preferred Stock is
automatically converted into shares of Acusphere's Common Stock, $.01 par value
per share (the "Common Stock"), pursuant to Section 4.2 of Part A of Article
Fourth of Acusphere's Certificate of Incorporation, as in effect from time to
time (the "Charter") (all of such shares of Common Stock into which the Series G
Preferred Stock is so automatically converted being referred to herein as the
"Series G Conversion Shares"), Elan shall be entitled to exchange all of such
Series G Conversion Shares for shares of Newco capital stock pursuant to the
Exchange Right set forth in Sections 1.1 and 1.2 hereto.

         1.4      In the event that any share of Series G Preferred Stock is
converted into shares of Common Stock at the option of Elan pursuant to Section
4.1 of Part A of Article Fourth of Acusphere's Charter, the Exchange Right
contained herein shall be terminated and of no further force and effect.

2.       MISCELLANEOUS.

         2.1      GOVERNING LAW. This Exchange Agreement shall be governed by
and construed under the laws of the Commonwealth of Massachusetts as applied to
agreements among Massachusetts residents, made and to be performed entirely
within the Commonwealth of Massachusetts.

         2.2      AMENDMENT. Any provision may be amended and the observance
thereof may be waived (either generally or in a particular instance and either
retroactively or prospectively), only by the written consent of the parties
hereto.

         2.3      ASSIGNMENT OF RIGHTS. This Exchange Agreement and the rights
and obligations of the parties hereunder shall inure to the benefit of, and be
binding upon, their respective successors, assigns and legal representatives;
PROVIDED, HOWEVER, that Elan shall not assign or transfer any part of its rights
and obligations hereunder without the express written consent of Acusphere other
than in connection with the transfer of all of its shares of Series G Preferred
Stock to any of its affiliated entities (including any off balance sheet special
purpose entity established by Elan).


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         2.4      TERM. This Exchange Agreement shall terminate upon the earlier
of (i) the date on which any share of Series G Preferred Stock is converted into
shares of Common Stock at the option of Elan pursuant to Section 4.1 of Part A
of Article Fourth of Acusphere's Charter, (ii) the date on which all of Elan's
shares of Series G Preferred Stock or Series G Conversion Shares, as the case
may be, are exchanged for such number of shares of Newco capital stock as is
required to be exchanged pursuant to Section 1.1 hereto, (iii) the date on which
Elan no longer owns, directly or indirectly through affiliated entities, any
shares of Series G Preferred Stock or, in the event the Series G Preferred Stock
is automatically converted into Series G Conversion Shares pursuant to Section
4.2 of Part A of Article Fourth of Acusphere's Charter, Series G Conversion
Shares, or (iv) the sixth anniversary of the date hereof.

         2.5      NOTICES. All notices required or permitted hereunder shall be
in writing and shall be deemed effectively given upon personal delivery to the
party to be notified or five (5) days after deposit in the United States mail,
by registered or certified mail, postage prepaid and properly addressed to the
party to be notified as set forth on the signature page hereof or at such other
address as such party may designate by ten (10) days advance written notice to
the other parties hereto.

         2.6      SEVERABILITY. In the event one or more of the provisions of
this Exchange Agreement should, for any reason, be held to be invalid, illegal
or unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provisions of this Exchange Agreement, and this
Exchange Agreement shall be construed as if such invalid, illegal or
unenforceable provision had never been contained herein.

         2.7      ATTORNEY FEES. In the event that any dispute among the parties
to this Exchange Agreement should result in litigation, the prevailing party in
such dispute shall be entitled to recover from the losing party all fees, costs
and expenses of enforcing any right of such prevailing party under or with
respect to this Exchange Agreement, including without limitation, such
reasonable fees and expenses of attorneys and accountants, which shall include,
without limitation, all fees, costs and expenses of appeals.

         2.8      COUNTERPARTS. This Exchange Agreement may be executed in two
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.


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         IN WITNESS WHEREOF, the parties have executed this Exchange Agreement
as of the date first above written.

                                    ACUSPHERE, INC.
                                    38 Sidney Street
                                    Cambridge, MA 02139

                                    By: /s/ Sherri C. Oberg
                                        ---------------------------------------
                                        Name:  Sherri C. Oberg
                                        Title: President and Chief Executive
                                               Officer


                                    ELAN INTERNATIONAL SERVICES, LTD.
                                    102 St. James Court
                                    Flatts, Smiths Parish
                                    Bermuda, FL 04


                                    By: /s/ Kevin Insley
                                        ---------------------------------------
                                        Name:
                                        Title: