Document:

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                                                                   Exhibit 10.25

                              ACUSPHERE NEWCO, LTD.
                          REGISTRATION RIGHTS AGREEMENT

         THIS REGISTRATION RIGHTS AGREEMENT (the "Agreement") is made as of June
30, 2000 by and among ACUSPHERE NEWCO, LTD., a Bermuda exempted limited
liability company (the "Company"), Acusphere, Inc., a Delaware corporation
("Acusphere"), and ELAN INTERNATIONAL SERVICES, LTD., a Bermuda exempted limited
liability company ("EIS").

                                R E C I T A L S:

                  A. Pursuant to a Subscription, Joint Development and Operating
Agreement dated as of the date hereof by and among the Company, Acusphere, ELAN
PHARMA INTERNATIONAL LIMITED, an Irish private limited company, ELAN
CORPORATION, plc, an Irish public limited company, acting through its division
ELAN PHARMACEUTICAL TECHNOLOGIES, and EIS (the "JDOA"), Acusphere has acquired
certain common shares, par value U.S.$1.00 per share (the "Common Shares"), and
certain non-voting convertible preferred shares, par value U.S.$1.00 per share
(the "Preferred Shares"), of the Company and EIS has acquired certain Preferred
Shares.

                  B. The execution of the JDOA has occurred on the date hereof
and it is a condition to the closing of the transactions contemplated thereby
that the parties execute and deliver this Agreement.

                  C. The parties desire to set forth herein their agreement on
the terms and subject to the conditions set forth herein related to the granting
of certain registration rights to the Holders (as defined below) relating to the
Common Shares held and the Common Shares issuable upon conversion of Preferred
Shares by such Holders.

                               A G R E E M E N T:

                  The parties hereto agree as follows:

                  1. Certain Definitions. As used in this Agreement, the
following terms shall have the following respective meanings:

                  "Commission" shall mean the U.S. Securities and Exchange
Commission.

                  "Exchange Act" shall mean the U.S. Securities Exchange Act of
1934, as amended, and the rules and regulations of the Commission thereunder,
all as the same shall be in effect from time to time.
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                                      -2-

                  "Holders" or "Holders of Registrable Securities" shall mean
Acusphere, EIS and any Person who shall have acquired Registrable Securities
from either Acusphere or EIS as permitted herein and in the JDOA, either
individually or jointly, as the case may be, in a transaction pursuant to which
registration rights are transferred pursuant to Section 10 hereof.

                  "Person" shall mean an individual, a partnership, a company,
an association, a joint stock company, a trust, a joint venture, an
unincorporated organization or a governmental or quasi-governmental entity, or
any department, agency or political subdivision thereof.

                  "Registrable Securities" means (i) any Common Shares
subscribed for pursuant to the JDOA; (ii) any Common Shares issued or issuable
upon conversion of the Preferred Shares; and (iii) any Common Shares issued or
issuable in respect of the securities referred to in clause (i) and (ii) above
upon any stock split, stock dividend, recapitalization or similar event, until,
in the case of any such security, it is effectively registered under the
Securities Act and disposed of in accordance with the registration statement
covering it; excluding in all cases, however, any Registrable Securities that
may be sold under Rule 144(k) promulgated under the Securities Act, as such rule
may be amended from time to time, or any similar rule or regulation hereafter
adopted by the Commission, and Registrable Securities sold by a Person in a
transaction (including a transaction pursuant to a registration statement under
this Agreement and a transaction pursuant to Rule 144 promulgated under the
Securities Act) in which registration rights are not transferred pursuant to
Section 10 hereof. Whenever a number or percentage of Registrable Securities is
to be determined pursuant to this Agreement, each then outstanding Preferred
Share that is convertible into Common Shares will be deemed to be equal to the
number of Common Shares for which such Preferred Share is then so convertible.

                  The terms "register," "registered" and "registration" refer to
a registration effected by preparing and filing a registration statement in
compliance with the Securities Act.

                  "Registration Expenses" shall mean all expenses, other than
Selling Expenses, incurred by the Company in complying with Sections 2 or 3
hereof, including without limitation, all registration, qualification and filing
fees, exchange listing fees, printing expenses, escrow fees, fees and
disbursements of counsel for the Company, blue sky fees and expenses, the
expense of any special audits incident to or required by any such registration.

                  "Securities Act" shall mean the Securities Act of 1933, as
amended.

                  "Selling Expenses" shall mean all underwriting discounts,
selling commissions and stock transfer taxes and the costs, fees and expenses of
any accountants, attorneys or other experts retained by the Holders.
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                                      -3-

                  2. Demand Registrations.

                  (a) Requests for Registration. Any Holder or Holders who
collectively hold Registrable Securities representing at least 5% of the
Registrable Securities then outstanding shall have the right (subject to the
limitations below) to request registration under the Securities Act of all or
part of their Registrable Securities on Form S-1 or S-3 (or any successor form
to Form S-1 or S-3, or any similar short-form registration statement) (each, a
"Demand Registration"). The request for the Demand Registration shall specify
the approximate number of Registrable Securities requested to be registered,
which must have a minimum expected aggregate offering price to the public of at
least U.S.$1,000,000. Within 10 days after receipt of any such request, the
Company will give written notice of such requested registration to all other
Holders of Registrable Securities. The Company shall include such other Holders'
Registrable Securities in such offering if they have responded affirmatively
within 10 days after the receipt of the Company's notice. Each of EIS and
Acusphere shall be permitted at least one Demand Registration hereunder;
provided, however, that the Holders in the aggregate shall be limited to three
Demand Registrations and will be entitled to request only one Demand
Registration in any 12-month period. A registration will not count as a
permitted Demand Registration until it has become effective (unless such Demand
Registration has not become effective due solely to the fault of the Holders
requesting such registration, including a request by such Holders that such
registration be withdrawn).

                  (b) Priority on Demand Registrations. If a Demand Registration
is an underwritten offering and the managing underwriters advise the Company in
writing that in their opinion the number of Registrable Securities and, if
permitted hereunder, other securities requested to be included in such offering,
exceeds the number of Registrable Securities and other securities, if any, which
can be sold in such offering without adversely affecting the marketability of
the offering the Company will include in such registration:

                  (i) first, the Registrable Securities requested to be included
         in such registration by the Holders (or, if necessary, such Registrable
         Securities pro rata among the Holders thereof based upon the number of
         Registrable Securities owned by each such Holder); and

                  (ii) thereafter, other securities requested to be included in
         such registration, as determined by the Company.

                  (c) Restrictions on Demand Registration. The Company may
postpone or suspend for up to three months in any 12-month period the filing or
the effectiveness of a registration statement for a Demand Registration if the
Company determines in good faith that such Demand Registration (i) would
reasonably be expected to have a material adverse effect on (x) any proposal or
plan by the Company to engage in any financing, acquisition or disposition of
assets (other than in the ordinary course of business) or (y) any merger,
consolidation, tender offer
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                                      -4-

or similar transaction or (ii) would require disclosure of any information that
the board of directors of the Company determines in good faith the disclosure of
which would be detrimental to the Company; provided, however, that in such
event, the Holders initially requesting such Demand Registration will be
entitled to withdraw such request and, if such request is withdrawn, such Demand
Registration will not count as a permitted Demand Registration hereunder and the
Company will pay any Registration Expenses in connection with such registration.

                  (d) Selection of Underwriters. The Holders will have the right
to select the investment banker(s) and manager(s) to administer an offering
pursuant to the Demand Registration, subject to the Company's prior written
approval, which will not be unreasonably withheld, delayed or conditioned.

                  (e) Other Registration Rights. Except as provided in this
Agreement, so long as any Holder owns any Registrable Securities, the Company
will not grant to any Persons the right to request the Company to register any
equity securities of the Company, or any securities convertible or exchangeable
into or exercisable for such securities, which conflicts with the rights granted
to the Holders hereunder, without the prior written consent of the Holders of at
least 50% of the Registrable Securities.

                  3. Piggyback Registrations.

                  (a) Right to Piggyback. If at any time the Company shall
propose to register Common Shares under the Securities Act (other than in a
registration statement on Form S-3 relating to sales of securities to
participants in a Company dividend reinvestment plan, or Form S-4 or S-8 or any
successor form or in connection with an acquisition or exchange offer or an
offering of securities solely to the existing shareholders or employees of the
Company) (each, a "Piggyback Registration"), the Company will (i) give prompt
written notice to all Holders of Registrable Securities of its intention to
effect such a registration and (ii) subject to Section 3(b) and the other terms
of this Agreement, will include in such registration all Registrable Securities
which are permitted under applicable securities laws to be included in the form
of registration statement selected by the Company and with respect to which the
Company has received written requests for inclusion therein within 10 days after
the receipt of the Company's notice. The Holders will be permitted to withdraw
all or any part of the Registrable Securities from a Piggyback Registration at
any time prior to the effective date of such Piggyback Registration.

                  (b) Priority on Piggyback Registrations. If a Piggyback
Registration is to be an underwritten offering, and the managing underwriters
advise the Company in writing that in their opinion the number of securities
requested to be included in such registration exceeds the number which can be
sold in such offering without adversely affecting the marketability of the
offering, the Company will include in such registration:

                  (i) first, the securities the Company proposes to sell;
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                                      -5-

                  (ii) second, the Registrable Securities requested to be
         included in such registration by the Holders and any securities
         requested to be included in such registration by any other Person other
         than Persons having a lower priority of registration than the Holders,
         pro rata among the Holders of such Registrable Securities and such
         other Persons, on the basis of the number of shares requested to be
         included in such registration by each of such Holders and such other
         Persons; and

                  (iii) thereafter, other securities requested to be included in
         such registration, as determined by the Company.

                  The Holders of any Registrable Securities included in such an
underwritten offering will execute an underwriting agreement in customary form
and in form and substance satisfactory to the managing underwriters.

                  (c) Right to Terminate Registration. If at any time after
giving written notice of its intention to register any of its securities as set
forth in Section 3(a) and prior to the effective date of the registration
statement filed in connection with such registration, the Company shall
determine for any reason not to register such securities, the Company may, at
its election, give written notice of such determination to each Holder of
Registrable Securities and thereupon be relieved of its obligation to register
any Registrable Securities in connection with such registration (but not from
its obligation to pay the Registration Expenses in connection therewith as
provided herein).

                  (d) Selection of Underwriters. The Company will have the right
to select the investment banker(s) and manager(s) to administer an offering
pursuant to a Piggyback Registration, subject to the approval of the holders of
a majority of the Registrable Securities, which approval will not be
unreasonably withheld, delayed or conditioned.

                  4. Expenses of Registration. Except as otherwise provided
herein or as may otherwise be prohibited by applicable law, all Registration
Expenses incurred in connection with all registrations pursuant to Sections 2
and 3 hereof shall be borne by the Company; provided that, and notwithstanding
anything herein contained to the contrary, the Company shall not have any
obligation pursuant to the provisions hereof unless and until the Company is
able to satisfy (after taking into account such obligations) the requirements of
Section 39A (2A) of the Bermuda Companies Act of 1981 (or any successor
legislation). All Selling Expenses relating to securities registered on behalf
of the Holders of Registrable Securities shall be borne by such Holders.

                  5. Holdback Agreements.

                  (a) The Company agrees (i) not to effect any public sale or
distribution of its equity securities, or any securities convertible into or
exchangeable or exercisable for such securities, during the 10-day period prior
to, and during the 90-day period following, on the
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                                      -6-

effective date of any underwritten Demand Registration or any underwritten
Piggyback Registration (except as part of such underwritten registration or
pursuant to registration statements on Form S-4 or Form S-8 or any successor
form), unless the underwriters managing the registered public offering otherwise
agree, and (ii) to use reasonable efforts to cause its officers and directors
and each holder of at least 5% (on a fully-diluted basis) of its outstanding
Common Shares, or any securities convertible into or exchangeable or exercisable
for Common Shares, purchased from the Company at any time after the date of this
Agreement (other than in a registered public offering) to agree not to effect
any public sale or distribution (including sales pursuant to Rule 144) of any
such securities during such periods (except as part of such underwritten
registration, if otherwise permitted), unless the underwriters managing the
registered public offering otherwise agree.

                  (b) Each Holder agrees, if requested by the managing
underwriters in an underwritten offering of Common Stock or securities
convertible for Common Stock of the Company, not to effect any offer, sale,
distribution or transfer, including a sale pursuant to Rule 144 (or any similar
provision then effect) under the Securities Act (except as part of such
underwritten registration), during the 10-day period prior to, and during the
90-day period (or such shorter period as may be agreed to in writing by the
Company and the Holders of at least 50% of the Registrable Securities)
following, the effective date of such Registration Statement; provided, however,
that no Holder shall be required to enter into more than one such agreement in
any 12-month period.

                  6. Registration Procedures. Whenever the Holders of
Registrable Securities have requested that any Registrable Securities be
registered pursuant to this Agreement, the Company will use all reasonable
efforts to effect the registration and the sale of such Registrable Securities
in accordance with the intended method or methods of distribution thereof, and
pursuant thereto the Company will as expeditiously as possible:

                  (a) subject to Section 2(c) hereof, prepare and file with the
         Commission a registration statement on any appropriate form for which
         the Company qualifies with respect to such Registrable Securities and
         use all reasonable efforts to cause such registration statement to
         become effective (provided that before filing a registration statement
         or prospectus or any amendments or supplements thereto, the Company
         will (i) furnish to the counsel selected by the selling Holders copies
         of all such documents proposed to be filed, which documents will be
         subject to the review of such counsel, and (ii) notify each Holder of
         Registrable Securities covered by such registration of any stop order
         issued or threatened by the Commission);

                  (b) subject to Section 2(c) hereof, prepare and file with the
         Commission such amendments and supplements to such registration
         statement and the prospectus used in connection therewith as may be
         reasonably necessary to keep such registration statement effective for
         a period equal to the shorter of (i) six months and (ii) the time by
         which all
<PAGE>
                                      -7-

         securities covered by such registration statement have been sold, and
         comply with the provisions of the Securities Act with respect to the
         disposition of all securities covered by such registration statement
         during such period in accordance with the intended methods of
         disposition by the sellers thereof set forth in such registration
         statement;

                  (c) furnish to each seller of Registrable Securities such
         number of copies of such registration statement, each amendment and
         supplement thereto, the prospectus included in such registration
         statement (including each preliminary prospectus) and such other
         documents as such seller may reasonably request in order to facilitate
         the disposition of the Registrable Securities owned by such seller;

                  (d) use all reasonable efforts to register or qualify such
         Registrable Securities under the securities or blue sky laws of such
         jurisdictions as any seller reasonably requests and do any and all
         other acts and things which may be reasonably necessary or advisable to
         enable such seller to consummate the disposition in such jurisdictions
         of the Registrable Securities owned by such seller (provided that the
         Company will not be required to (i) qualify generally to do business in
         any jurisdiction where it would not otherwise be required to qualify
         but for this Section 6(d), (ii) subject itself to taxation in any
         jurisdiction or (iii) take any action that would subject it to general
         service of process in any such jurisdiction);

                  (e) notify each seller of such Registrable Securities, at any
         time when a prospectus relating thereto is required to be delivered
         under the Securities Act, of the happening of any event as a result of
         which the prospectus included in such registration statement contains
         an untrue statement of a material fact or omits any fact necessary to
         make the statements therein not misleading, and, at the request of any
         such seller, the Company will prepare a supplement or amendment to such
         prospectus so that, as thereafter delivered to the purchasers of such
         Registrable Securities, such prospectus will not contain an untrue
         statement of a material fact or omit to state any fact necessary to
         make the statements therein not misleading; provided, however, that the
         Company shall not be required to amend the registration statement or
         supplement the Prospectus for a period of up to six months if the board
         of directors determines in good faith that to do so would reasonably be
         expected to have a material adverse effect on any proposal or plan by
         the Company to engage in any financing, acquisition or disposition of
         assets (other than in the ordinary course of business) or any merger,
         consolidation, tender offer or similar transaction or would require the
         disclosure of any information that the board of directors determines in
         good faith the disclosure of which would be detrimental to the Company,
         it being understood that the period for which the Company is obligated
         to keep the Registration Statement effective shall be extended for a
         number of days equal to the number of days the Company delays
         amendments or supplements pursuant to this provision. Upon receipt of
         any notice pursuant to this Section 6(e), the Holders shall suspend all
         offers and
<PAGE>
                                      -8-

         sales of securities of the Company and all use of any prospectus until
         advised by the Company that offers and sales may resume, and shall keep
         confidential the fact and content of any notice given by the Company
         pursuant to this Section 6(e);

                  (f) cause all such Registrable Securities to be listed on each
         securities exchange, if any, on which similar securities issued by the
         Company are then listed;

                  (g) provide a transfer agent and registrar for all such
         Registrable Securities not later than the effective date of such
         registration statement;

                  (h) enter into such customary agreements (including
         underwriting agreements in customary form) and take all such other
         actions as the Holders of a majority of the Registrable Securities
         being sold or the underwriters, if any, reasonably request in order to
         expedite or facilitate the disposition of such Registrable Securities;

                  (i) make available for inspection by a representative of the
         Holders of Registrable Securities included in the registration
         statement, any underwriter participating in any disposition pursuant to
         such registration statement and any attorney, accountant or other agent
         retained by any such seller or underwriter, all pertinent financial and
         other records, pertinent corporate documents and properties of the
         Company, and cause the Company's officers, directors, employees and
         independent accountants to supply all information reasonably requested
         by any such seller, underwriter, attorney, accountant or agent in
         connection with such registration statement;

                  (j) otherwise use its reasonable efforts to comply with all
         applicable rules and regulations of the Commission, and make available
         to its security holders, as soon as reasonably practicable, an earnings
         statement covering the period of at least 12 months beginning with the
         first day of the Company's first full calendar quarter after the
         effective date of the registration statement, which earnings statement
         shall satisfy the provisions of Section 11(a) of the Securities Act
         and Rule 158 thereunder;

                  (k) in the event of the issuance of any stop order suspending
         the effectiveness of a registration statement, or of any order
         suspending or preventing the use of any related prospectus or
         suspending the qualification of any Common Shares included in such
         registration statement for sale in any jurisdiction, use all reasonable
         efforts promptly to obtain the withdrawal of such order; and

                  (1) if the registration is an underwritten offering, use all
         reasonable efforts to obtain a so-called "cold comfort" letter from the
         Company's independent public accountants in customary form and covering
         such matters of the type customarily covered by cold comfort letters.
<PAGE>
                                      -9-

                  7. Obligations of Holders. Whenever the Holders of Registrable
Securities sell any Registrable Securities pursuant to a Demand Registration or
a Piggyback Registration, such Holders shall be obligated to comply with the
applicable provisions of the Securities Act, including the prospectus delivery
requirements thereunder, and any applicable state securities or blue sky laws.
In addition, each Holder of Registrable Securities will be deemed to have agreed
by virtue of its acquisition of such Registrable Securities that, upon receipt
of any notice described in Section 6(e), such holder will forthwith discontinue
disposition of such Registrable Securities covered by such registration
statement or prospectus until such holder's receipt of the copies of the
supplemented or amended prospectus contemplated by Section 6(e), or until it is
advised in writing by the Company that the use of the applicable prospectus may
be resumed, and has received copies of any additional or supplemental filings
that are incorporated or deemed to be incorporated by reference in such
prospectus.

                  8. Indemnification.

                  (a) The Company agrees to indemnify, to the fullest extent
permitted by applicable law, each Holder of Registrable Securities, its officers
and directors and each Person who controls such Holder (within the meaning of
the Securities Act) against all losses, claims, damages, liabilities, expenses
or any amounts paid in settlement of any litigation, investigation or proceeding
commenced or threatened (collectively, "Claims") to which each such indemnified
party may become subject under the Securities Act insofar as such Claim arose
out of (i) any untrue or alleged untrue statement of material fact contained, on
the effective date thereof, in any registration statement, prospectus or
preliminary prospectus or any amendment thereof or supplement thereto or (ii)
any omission or alleged omission to state therein a material fact required to be
stated therein or necessary to make the statements therein not misleading,
except insofar as the same are caused by or contained in any information
furnished in writing to the Company by such Holder expressly for use therein, by
such Holder's failure to deliver a copy of the registration statement or
prospectus or any amendments or supplements thereto after the Company has
furnished such Holder with a sufficient number of copies of the same or by such
Holder's failure to comply with applicable securities laws. In connection with
an underwritten offering, the Company will indemnify the underwriters, their
officers and directors and each Person who controls the underwriters (within the
meaning of the Securities Act) to the same extent as provided above with
respect to the indemnification of the Holders of Registrable Securities.

                  (b) In connection with any registration statements in which a
Holder of Registrable Securities is participating, each such Holder will, to the
fullest extent permitted by applicable law, indemnify the Company, its directors
and officers and each Person who controls the Company (within the meaning of the
Securities Act) against any and all Claims to which each such indemnified party
may become subject under the Securities Act insofar as such Claim arose out of
(i) any untrue or alleged untrue statement of material fact contained, on the
effective date thereof, in any registration statement, prospectus or preliminary
prospectus or any amendment
<PAGE>
                                      -10-

thereof or supplement thereto, (ii) any omission or alleged omission to state
therein a material fact required to be stated therein or necessary to make the
statements therein not misleading or (iii) any failure to comply with applicable
securities laws; provided that with respect to a Claim arising pursuant to
clause (i) or (ii) above, the material misstatement or omission is contained in
the information such Holder provided to the Company pursuant to Section 11
hereof; provided, further, that the obligation to indemnify will be individual
to each Holder and will be limited to the amount of proceeds received by such
Holder from the sale of Registrable Securities pursuant to such registration
statement.

                  (c) Any Person entitled to indemnification hereunder will (i)
give prompt written notice to the indemnifying party of any claim with respect
to which it seeks indemnification (but the failure to provide such notice shall
not release the indemnifying party of its obligation under paragraphs (a) and
(b), unless and then only to the extent that, the indemnifying party has been
prejudiced by such failure to provide such notice) and (ii) unless in such
indemnified party's reasonable judgment, based on written advice of counsel, a
conflict of interest between such indemnified and indemnifying parties may exist
with respect to such claim, permit such indemnifying party to assume the defense
of such claim with counsel reasonably satisfactory to the indemnified party. An
indemnifying party who is not entitled to, or elects not to, assume the defense
of a claim will not be obligated to pay the fees and expenses of more than one
counsel for all parties indemnified by such indemnifying party with respect to
such claim, unless in the reasonable judgment of any indemnified party, based on
written advice of counsel, a conflict of interest may exist between such
indemnified party and any other of such indemnified parties with respect to such
claim.

                  (d) The indemnifying party shall not be liable to indemnify an
indemnified party for any settlement, or consent to judgment of any such action
effected without the indemnifying party's written consent (but such consent will
not be unreasonably withheld, delayed or conditioned). Furthermore, the
indemnifying party shall not, except with the prior written approval of each
indemnified party, consent to entry of any judgment or enter into any settlement
which does not include as an unconditional term thereof the giving by the
claimant or plaintiff to each indemnified party of a release from all liability
in respect of such claim or litigation without any payment or consideration
provided by each such indemnified party.

                  (e) If the indemnification provided for in this Section 8 is
unavailable to an indemnified party under clauses (a) and (b) above in respect
of any losses, claims, damages or liabilities referred to therein, then each
indemnifying party, in lieu of indemnifying such indemnified party, shall
contribute to the amount paid or payable by such indemnified party as a result
of such losses, claims, damages or liabilities in such proportion as is
appropriate to reflect not only the relative benefits received by the Company,
the underwriters, the sellers of Registrable Securities and any other sellers
participating in the registration statement from the sale of shares pursuant to
the registered offering of securities for which indemnity is sought but also the
relative
<PAGE>
                                      -11-

fault of the Company, the underwriters, the sellers of Registrable Securities
and any other sellers participating in the registration statement in connection
with the misstatement or omission which resulted in such losses, claims, damages
or liabilities, as well as any other relevant equitable considerations. The
relative benefits received by the Company, the underwriters, the sellers of
Registrable Securities and any other sellers participating in the registration
statement shall be deemed to be based on the relative relationship of the total
net proceeds from the offering (before deducting expenses) to the Company, the
total underwriting commissions and fees from the offering (before deducting
expenses) to the underwriters and the total net proceeds from the offering
(before deducting expenses) to the sellers of Registrable Securities and any
other sellers participating in the registration statement. The relative fault of
the Company, the underwriters, the sellers of Registrable Securities and any
other sellers participating in the registration statement shall be determined by
reference to, among other things, whether the untrue or alleged untrue statement
of a material fact or the omission or alleged omission to state a material fact
relates to information supplied by the Company or by the sellers of Registrable
Securities and the parties' relative intent, knowledge, access to information
and opportunity to correct or prevent such statement or omission; provided that
in no event shall the liability of any selling Holder hereunder be greater in
amount than the dollar amount of the proceeds received by such Holder upon the
sale of the Registrable Securities giving rise to such indemnification
obligation.

                  (f) The indemnification provided for under this Agreement will
remain in full force and effect regardless of any investigation made by or on
behalf of the indemnified party or any officer, director or controlling person
of such indemnified party and will survive the transfer of the Registrable
Securities.

                  9. Participation in Underwritten Registrations. No Holder may
participate in any registration hereunder which is underwritten unless such
Holder (a) agrees to sell such Holder's securities on the basis provided in any
underwriting arrangements approved by the Holder or Holders entitled hereunder
to approve such arrangements, (b) as expeditiously as possible notifies the
Company of the occurrence of any event as a result of which any prospectus
contains an untrue statement of material fact or omits to state a material fact
required to be stated therein or necessary to make the statements therein not
misleading and (c) completes and executes all questionnaires, powers of
attorney, indemnities, underwriting agreements and other documents reasonably
required under the terms of such underwriting arrangements.

                  10. Transfer of Registration Rights. The rights granted to any
Holder under this Agreement may be assigned to any permitted transferee of
Registrable Securities, in connection with any transfer or assignment of
Registrable Securities by a Holder; provided, however, that: (a) such transfer
is otherwise effected in accordance with applicable securities laws, (b) such
transfer is permitted by the JDOA, (c) if not already a party hereto, the
assignee or transferee agrees in writing prior to such transfer to be bound by
the provisions of this Agreement applicable
<PAGE>
                                      -12-

to the transferor and (d) Acusphere or EIS, as applicable, if a transferor,
shall act as agent and representative for such Holder for the giving and
receiving of notices hereunder.

                  11. Information by Holder. Each Holder shall furnish to the
Company such written information regarding such Holder and any distribution
proposed by such Holder as the Company may reasonably request in writing and as
shall be reasonably required in connection with any registration, qualification
or compliance referred to in this Agreement and shall promptly notify the
Company of any changes in such information.

                  12. Exchange Act Compliance. The Company shall comply with all
of the reporting requirements of the Exchange Act then applicable to it, if any,
and shall comply with all other public information reporting requirements of the
Commission which are conditions to the availability of Rule 144 for the sale of
the Registrable Securities. The Company shall cooperate with each Holder in
supplying such information as may be necessary for such Holder to complete and
file any information reporting forms presently or hereafter required by the
Commission as a condition to the availability of Rule 144.

                  13. Termination of Registration Rights. All registration
rights and obligations (including, without limitation, under Section 5) under
this Agreement shall terminate and be of no further force and effect, as to any
particular Holder, at such time as all Registrable Securities held by such
Holder are eligible to be sold without compliance with the registration
requirements of the Securities Act pursuant to Rule 144(k) promulgated
thereunder or have been resold pursuant to a registration statement hereunder.

                  14. Miscellaneous.

                  (a) No Inconsistent Agreements. The Company will not hereafter
enter into any agreement with respect to its securities which is inconsistent
with or violates the rights granted to the Holders of Registrable Securities in
this Agreement without the prior written consent of a majority in interest of
such Registrable Securities.

                  (b) Remedies. Any Person having rights under any provision of
this Agreement will be entitled to enforce such rights specifically to recover
damages caused by reason of any breach of any provision of this Agreement and
to exercise all other rights granted by law. The parties hereto agree and
acknowledge that money damages may not be an adequate remedy for any breach of
the provisions of this Agreement and that any party may in its sole discretion
apply to any court of law or equity of competent jurisdiction (without posting
any bond or other security) for specific performance and for other injunctive
relief in order to enforce or prevent violation of the provisions of this
Agreement; provided, however, that in no event shall any Holder have the right
to enjoin, delay or interfere with any offering of securities by the Company.
<PAGE>
                                      -13-

                  (c) Amendments and Waivers. Except as otherwise provided
herein, the provisions of this Agreement may be amended or waived only with the
prior written consent of the Company and Holders of at least 50% of the
Registrable Securities; provided, however, that without the prior written
consent of all the Holders, no such amendment or waiver shall reduce the
foregoing percentage required to amend or waive any provision of this Agreement.

                  (d) Successors and Assigns. This Agreement and all of the
provisions hereof shall be binding upon and inure to the benefit of the parties
and their respective successors and permitted assigns. This Agreement may be
transferred by EIS and its permitted assigns and transferees to their respective
affiliates, as well as any special purpose financing or similar vehicle
established by EIS. Other than as set forth above, no party shall transfer or
assign this Agreement without the prior written consent of the other party,
which will not be unreasonably withheld, delayed or conditioned. In addition,
whether or not any express assignment has been made, the provisions of this
Agreement which are for the benefit of Holders of Registrable Securities are
also for the benefit of, and enforceable by, any permitted transferee of
Registrable Securities, in accordance with Section 10 hereof.

                  (e) Severability. In case any provision of this Agreement
shall be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions shall not be in any way affected or
impaired thereby.

                  (f) Counterparts and Facsimile. This Agreement may be executed
in any number of counterparts, and each such counterpart hereof shall be deemed
to be an original instrument, but all such counterparts together shall
constitute one agreement. This Agreement may be signed and delivered to the
other party by facsimile transmission; such transmission shall be deemed a valid
signature.

                  (g) Descriptive Headings. The section and paragraph headings
contained in this Agreement are for reference purposes only and shall not affect
in any way the meaning or interpretation of this Agreement.

                  (h) Governing Law; Disputes. This Agreement shall be governed
by and construed in accordance with the internal laws of the State of New York,
without giving effect to principles of conflicts of laws. Any dispute under this
Agreement that is not settled by mutual consent shall be finally adjudicated by
any federal or state court sitting in the City, County and State of New York,
and each party consents to the exclusive jurisdiction of such courts (or any
appellate court therefrom) over any such dispute.

                  (i) Notices. All notices, demands and requests of any kind to
be delivered to any party in connection with this Agreement shall be in writing
and shall be deemed to have been duly given if personally or hand delivered or
if sent by internationally-recognized overnight courier
<PAGE>
                                      -14-

or by registered or certified mail, return receipt requested and postage
prepaid, or by facsimile transmission, addressed as follows:

         (i)      if to the Company, to:

                  Acusphere Newco, Ltd.
                  c/o Conyers Dill & Pearman
                  Clarendon House
                  Church Street, P.O. Box HM 666
                  Hamilton HM CX, Bermuda
                  Attention: David J. Doyle
                  Facsimile: (441) 292-4720

                  with a copy to each of Acusphere, EIS and their respective
                  counsel at the addresses indicated below

         (ii)     if to Acusphere, to:

                  Acusphere, Inc.
                  University Park at M.I.T.
                  38 Sidney Street
                  Cambridge, MA 02139
                  Attention:        Steven W. Delaney, Vice President
                                    Finance and Administration

                  Facsimile: 617-577-0233
<PAGE>
                                      -15-

                  with a copy to:

                  Testa, Hurwitz & Thibeault, LLP
                  High Street Tower
                  125 High Street
                  Boston, MA 02110
                  Attention: Lawrence Wittenberg
                  Facsimile: 617-248-7100

        (iii)     if to EIS, to:

                  Elan International Services, Ltd.
                  Flatts, Smiths Parish
                  Bermuda FL 04
                  Attention: Chief Executive Officer
                  Facsimile: (441) 292-2224

                  with a copy to:

                  Cahill Gordon & Reindel
                  80 Pine Street
                  New York, NY 10005
                  Attention: William M. Hartnett
                  Facsimile: 212-269-5480

or to such other address as the party to whom notice is to be given may have
furnished to the other party hereto in writing in accordance with provisions of
this Section 14(i). Any such notice or communication shall be deemed to have
been effectively given (i) in the case of personal or hand delivery, on the date
of such delivery, (ii) in the case of an internationally-recognized overnight
delivery courier, on the second business day after the date when sent, (iii) in
the case of mailing, on the fifth business day following that day on which the
piece of mail containing such communication is posted and (iv) in the case of
facsimile transmission, on the date of telephone confirmation of receipt.

                  (j) Entire Agreement. This Agreement constitutes the full and
entire understanding and agreement of the parties with regard to the subject
matter hereof and supersedes all prior agreements and understandings among the
parties with respect thereto.

                            [Signature page follows]
<PAGE>
         IN WITNESS WHEREOF, the parties have executed this Registration Rights
Agreement as of the date first written above.

                                       ACUSPHERE NEWCO, LTD.

                                       By: /s/ Stephen Rossiter
                                          --------------------------------------
                                          Name:  Stephen Rossiter
                                          Title: Director

                                       ACUSPHERE, INC.

                                       By: /s/ Sherri C. Oberg
                                          --------------------------------------
                                          Name:  Sherri C. Oberg
                                          Title: President and C.E.O.

                                       ELAN INTERNATIONAL SERVICES, LTD.

                                       By: /s/ Kevin Insley
                                          --------------------------------------
                                          Name: Kevin Insley
                                          Title:<PAGE>
                                                                   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

<PAGE>

                                      -2-

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).

<PAGE>
                                      -3-

         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.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

<PAGE>

         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:

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