<PAGE>
                                                                   EXHIBIT 10.07

             TENTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

                                 ACUSPHERE, INC.

                                 April 11, 2003
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                                TABLE OF CONTENTS

<TABLE>
<S>      <C>                                                                                                     <C>
1.    TERMINATION OF NINTH AMENDED INVESTORS' RIGHTS AGREEMENT; WAIVER AND CONSENT................................2
   1.1   Termination of Ninth Amended Investors' Rights Agreement.................................................2
   1.2   Waiver and Consent.......................................................................................2


2.    REGISTRATION RIGHTS.........................................................................................2
   2.1   Definitions..............................................................................................2
   2.2   Demand Registration......................................................................................5
   2.3   "Piggy-Back" Registration................................................................................6
   2.4   Form S-3 Registration....................................................................................7
   2.5   Obligations of Acusphere.................................................................................8
   2.6   Furnish Information.....................................................................................10
   2.7   Expenses of Demand and S-3 Registrations................................................................10
   2.8   Expenses of "Piggy-Back" Registration...................................................................11
   2.9   Delay of Registration...................................................................................11
   2.10  Indemnification.........................................................................................11
   2.11  Reports Under Securities Exchange Act of 1934...........................................................13
   2.12  Assignment of Registration Rights.......................................................................13
   2.13  Limitations on Subsequent Registration Rights...........................................................14
   2.14  "Market Stand-Off" Agreement............................................................................14
   2.15  Termination of Registration Rights......................................................................15


3.    COVENANTS OF ACUSPHERE.....................................................................................15
   3.1   Delivery of Financial Statements........................................................................15
   3.2   Inspection and Observation..............................................................................16
   3.3   Termination of Information, Inspection and Observation Covenants, Assignment............................16
   3.4   Right of First Refusal..................................................................................17
   3.5   Preparation of Audited Financial Statements.............................................................20
   3.6   Stock Purchase Agreements...............................................................................20
   3.7   Termination of Certain Covenants........................................................................20
   3.8   Covenants Relating to SBA Matters.......................................................................20
   3.9   Regulatory Compliance Cooperation.......................................................................21
   3.10  Consideration of Section 1202(c)........................................................................22
   3.11  Registration Rights.....................................................................................22


4.    MISCELLANEOUS..............................................................................................22
   4.1   Successors and Assigns..................................................................................22
   4.2   Governing Law...........................................................................................22
   4.3   Counterparts............................................................................................22
   4.4   Titles and Subtitles....................................................................................22
</TABLE>
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                                     - ii -

<TABLE>
<S>      <C>                                                                                                     <C>
   4.5   Notices.................................................................................................22
   4.6   Expenses................................................................................................23
   4.7   Amendments and Waivers..................................................................................23
   4.8   Severability............................................................................................23
   4.9   Aggregation of Stock....................................................................................23
   4.10  Entire Agreement........................................................................................23
</TABLE>
<PAGE>
             TENTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT

      THIS TENTH AMENDED AND RESTATED INVESTORS' RIGHTS AGREEMENT (this
"Agreement") is made as of the 11th day of April, 2003, by and among Acusphere,
Inc., a Delaware corporation ("Acusphere"), and those investors listed on
Schedule A attached hereto (the "Investors"); and, solely for purposes of
Sections 1, 3.4 and 4 of this Agreement, Sherri C. Oberg ("Oberg").

                                    RECITALS

      WHEREAS, certain of the Investors hold shares of Acusphere's Series A-1
Convertible Preferred Stock, $.01 par value per share (the "Series A-1 Stock"),
Series A-2 Convertible Preferred Stock, $.01 par value per share (the "Series
A-2 Stock"), Series B-1 Convertible Preferred Stock, $.01 par value per share
(the "Series B-1 Stock"), Series B-2 Convertible Preferred Stock, $.01 par value
per share (the "Series B-2 Stock"), Series C-1 Convertible Preferred Stock, $.01
par value per share (the "Series C-1 Stock"), Series C-2 Convertible Preferred
Stock, $.01 par value per share (the "Series C-2 Stock"), Series D-1 Convertible
Preferred Stock, $.01 par value per share (the "Series D-1 Stock"), Series D-2
Convertible Preferred Stock, $.01 par value per share (the "Series D-2 Stock"),
Series E-1 Convertible Preferred Stock, $.01 par value per share (the "Series
E-1 Stock"), Series E-2 Convertible Preferred Stock, $.01 par value per share
(the "Series E-2 Stock"), Series F-1 Convertible Preferred Stock, $.01 par value
per share (the "Series F-1 Stock"), Series F-2 Convertible Preferred Stock, $.01
par value per share (the "Series F-2 Stock"), Series I-1 Convertible Preferred
Stock, $.01 par value share (the "Series I-1 Stock"), Series I-2 Convertible
Preferred Stock, $.01 par value per share (the "Series I-2 Stock"), Series J
Convertible Preferred Stock, $.01 par value per share (the "Series J Stock"),
Series J-1 Convertible Preferred Stock, $.01 par value per share (the "Series
J-1 Stock"), Series J-2 Convertible Preferred Stock, $.01 par value per share
(the "Series J-2 Stock"), and/or Series J-3 Convertible Preferred Stock, $.01
par value per share (the "Series J-3 Stock," together with the Series A-1 Stock,
Series A-2 Stock, Series B-1 Stock, Series B-2 Stock, Series C-1 Stock, Series
C-2 Stock, Series D-1 Stock, Series D-2 Stock, Series E-1 Stock, Series E-2
Stock, Series F-1 Stock, Series F-2 Stock, Series I-1 Stock, Series I-2 Stock,
Series J Stock, Series J-1 Stock and Series J-2 Stock, the "Preferred Stock");

      WHEREAS, a certain Investor, Alexandria Real Estate Equities, L.P.
("Alexandria"), holds a warrant to purchase (i) prior to 12:00 midnight, Boston
time, on July 1, 2003 shares of Series F-1 Non-Voting Convertible Preferred
Stock, $.01 par value per share (the "Series F-1 Non-Voting Stock"), and (ii)
from and following such time, shares of Series F-2 Non-Voting Convertible
Preferred Stock, $.01 par value per share (the "Series F-2 Non-Voting Stock")
(the "Alexandria Warrant Shares");

      WHEREAS, certain Investors, GATX Ventures, Inc. ("GATX") and Venture
Lending & Leasing III, LLC ("VLL"), hold warrants to purchase (i) prior to 12:00
midnight, Boston time, on July 1, 2003 shares of Series F-1 Non-Voting Stock,
and (ii) from and following such time, shares of shares of Series F-2 Non-Voting
Stock (the "Venture Lender Warrant Shares");
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                                      -2-

      WHEREAS, each such Investor possesses certain registration and other
rights in connection therewith pursuant to a Ninth Amended and Restated
Investors' Rights Agreement, dated as of June 17, 2002, by and among Acusphere
and such Investors, (the "Ninth Amended Investors' Rights Agreement");

      WHEREAS, certain Investors are parties to a Convertible Note and Warrant
Purchase and Securities Exchange Agreement, dated as of the date hereof (as
amended and in effect from time to time, the "Note and Warrant Purchase
Agreement"), which provides, inter alia, (i) for the purchase by such Investors
of 10% convertible promissory notes (the "Notes") and warrants to purchase
shares of Acusphere's equity securities (the "Warrants") and (ii) for the
exchange of certain shares of Existing Preferred (as defined therein) for shares
of New Preferred (as defined therein); and

      WHEREAS, among the conditions to the consummation of the transactions
contemplated by the Note and Warrant Purchase Agreement is the execution and
delivery of this Agreement; and

      WHEREAS, each of the parties hereto desires to set forth in a single
document such registration and certain other rights of the Investors.

      NOW, THEREFORE, in consideration of the mutual promises, covenants and
conditions hereinafter set forth and for other good and valuable consideration,
Acusphere, each Investor and Oberg, severally and not jointly, hereby agree as
follows:

1. Termination of Ninth Amended Investors' Rights Agreement; Waiver and Consent.

      1.1 Termination of Ninth Amended Investors' Rights Agreement. The parties
hereto hereby acknowledge and agree that the Ninth Amended Investors' Rights
Agreement is hereby amended, restated and superseded in all respects by this
Agreement.

      1.2 Waiver and Consent. The Investors and all other parties to the Ninth
Amended Investors' Rights Agreement hereby (i) consent to the issuance of the
Notes, Warrants and New Preferred in accordance with the terms of the Note and
Warrant Purchase Agreement (including all closings described therein) and (ii)
waive all preemptive rights and rights of first refusal, and all related notice
provisions, under all existing agreements with respect to the issuance by
Acusphere of such Notes, Warrants, and New Preferred, including any shares of
capital stock issued or issuable upon conversion, exercise, or exchange (as
applicable) of such securities, and the shares of Common Stock, if any, issued
or issuable upon conversion thereof.

2. Registration Rights. Acusphere covenants and agrees as follows:

      2.1 Definitions. For purposes of this Section 2:

            (a) The term "Act" means the Securities Act of 1933, as amended.

            (b) The term "Alexandria Registrable Securities" means (i) those
shares of Common Stock issued or
<PAGE>
                                      -3-

issuable upon conversion of the Alexandria Warrant Shares issued or issuable
upon exercise of that certain Warrant Agreement by and between Acusphere and
Alexandria, dated March 30, 2001 (the "Alexandria Warrant") and (ii) any common
stock of Acusphere issued as (or issuable upon the conversion or exercise of any
warrant, right or other security which is issued as) a dividend or other
distribution with respect to, or in exchange for or in replacement of the shares
referenced in (i) above, excluding in all cases, however, any Registrable
Securities sold by a person in a transaction in which the rights under this
Section 2 are not assigned.

            (c) The term "Bridge Note Registrable Securities" means (i) those
shares of Common Stock issued or issuable upon conversion of those shares of
Acusphere capital stock, if any, issued upon conversion or exchange of any Notes
issued pursuant to the Note and Warrant Purchase Agreement, and (ii) any shares
of common stock of Acusphere issued as (or issuable upon the conversion or
exercise of any warrant, right or other security which is issued as) a dividend
or other distribution with respect to, or in exchange for or in replacement of
the shares referenced in (i) above, excluding in all cases, however, any Bridge
Note Registrable Securities sold by a person in a transaction in which the
rights under this Section 2 are not assigned.

            (d) The term "Bridge Warrant Registrable Securities" means (i) those
shares of Common Stock issued or issuable upon conversion of those shares of
Acusphere capital stock issued or then issuable upon exercise of any Warrants
issued pursuant to the Note and Warrant Purchase Agreement, and (ii) any shares
of common stock of Acusphere issued as (or issuable upon the conversion or
exercise of any warrant, right or other security which is issued as) a dividend
or other distribution with respect to, or in exchange for or in replacement of
the shares referenced in (i) above, excluding in all cases, however, any Bridge
Warrant Registrable Securities sold by a person in a transaction in which the
rights under this Section 2 are not assigned.

            (e) The term "Common Stock" means shares of Common Stock, $.0l par
value per share, of Acusphere.

            (f) The term "Form S-1" means such form under the Act as in effect
on the date hereof, or any registration form under the Act subsequently adopted
by the SEC which permits the registration of securities under the Act for which
no other form is authorized or prescribed.

            (g) The term "Form S-3" means such form under the Act as in effect
on the date hereof or any registration form under the Act subsequently adopted
by the SEC which permits inclusion or incorporation of substantial information
by reference to other documents filed by Acusphere with the SEC.

            (h) The term "Holder" means any person owning or having the right to
acquire Registrable Securities or any assignee thereof in accordance with
Section 2.12 hereof.

            (i) The term "1934 Act" shall mean the Securities Exchange Act of
1934, as amended.
<PAGE>
                                      -4-

            (j) The terms "register," "registered," and "registration" refer to
a registration effected by preparing and filing a registration statement or
similar document in compliance with the Act, and the declaration or ordering of
effectiveness of such registration statement or document.

            (k) The term "Registrable Securities" means (A) (i) the Common Stock
issuable or issued upon conversion of the Series A-1 Stock, the Series A-2
Stock, the Series B-1 Stock, the Series B-2 Stock, the Series C-1 Stock, the
Series C-2 Stock, the Series D-1 Stock, the Series D-2 Stock, the Series E-1
Stock, the Series E-2 Stock, the Series F-1 Stock, the Series F-2 Stock, the
Series I-1 Stock, the Series I-2 Stock, the Series J Stock, the Series J-1
Stock, the Series J-2 Stock and the Series J-3 Stock, (ii) any Common Stock of
Acusphere issued as (or issuable upon the conversion or exercise of any warrant,
right or other security which is issued as) a dividend or other distribution
with respect to, or in exchange for or in replacement of the shares referenced
in (i) above, (iii) the Bridge Note Registrable Securities, (iv) the Bridge
Warrant Registrable Securities, (v) the Alexandria Registrable Securities, and
(vi) the Venture Lender Registrable Securities, excluding in all cases, however,
any Registrable Securities sold by a person in a transaction in which the rights
under this Section 2 are not assigned, and (B) solely for the purposes of
Sections 2.1, 2.3 through 2.15, 3.11, 4.1, 4.7 and 4.9 of this Agreement, the
Alexandria Registrable Securities and the Venture Lender Registrable Securities.

            (l) The number of shares of "Registrable Securities then
outstanding" shall be determined by the number of shares of Common Stock
outstanding which are, and the number of shares of Common Stock issuable
pursuant to then exercisable or convertible securities which are, Registrable
Securities.

            (m) The term "SEC" means the Securities and Exchange Commission.(n)
The term "SBIC Investor" means any Investor that is a Small Business Investment
Company, as defined in 13 C.F.R. Section 107.

            (o) The term "Venture Lender Registrable Securities" means (i) those
shares of Common Stock issued or issuable upon conversion of the Venture Lender
Warrant Shares issued or issuable upon exercise of that certain Warrant
Agreement by and between Acusphere and GATX, dated as of September 27, 2001 (the
"GATX Warrant"), and that certain Warrant Agreement by and between Acusphere and
VLL, dated as of September 27, 2001 (the "VLL Warrant"), and (ii) any common
stock of Acusphere issued as (or issuable upon the conversion or exercise of any
warrant, right or other security which is issued as) a dividend or other
distribution with respect to, or in exchange for or in replacement of the shares
referenced in (i) above, excluding in all cases, however, any Registrable
Securities sold by a person in a transaction in which the rights under this
Section 2 are not assigned.

      2.2 Demand Registration.

            (a) If Acusphere shall receive at any time after the earlier of (i)
June 17, 2005 or (ii) six months after the closing of an initial public offering
of securities of Acusphere (other than a registration statement relating either
to the sale of securities to employees of Acusphere pursuant to a stock option,
stock purchase or similar plan or a SEC Rule 145 transaction), a
<PAGE>
                                      -5-

written request from (i) the Holders of at least forty percent (40%) of the
Registrable Securities then outstanding, (ii) the Holders of at least a majority
of the Series F-1 Stock, Series F-2 Stock, Series I-1 Stock, Series I-2 Stock,
Series J Stock, Series J-1 Stock, Series J-2 Stock and Series J-3 Stock then
outstanding (including, for such purposes, any shares of Common Stock issued
upon conversion thereof), that Acusphere effect a registration statement under
the Act with respect to all or a part of the Registrable Securities, then
Acusphere shall:

                  (i) within ten (10) days of the receipt thereof, give written
      notice of such request to all Holders; and

                  (ii) effect as soon as practicable, and in any event within
      ninety (90) days of the receipt of such request, the registration under
      the Act of all Registrable Securities which the Holders request to be
      registered, subject to the limitations of subsection 2.2(b), within thirty
      (30) days of the mailing of such notice by Acusphere in accordance with
      Section 4.5.

            (b) If the Holders initiating the registration request hereunder
(the "Initiating Holders") intend to distribute the Registrable Securities
covered by their request by means of an underwriting, they shall so advise
Acusphere as a part of their request made pursuant to subsection 2.2(a) and
Acusphere shall include such information in the written notice referred to in
subsection 2.2(a). The underwriter will be selected by a majority in interest of
the Initiating Holders, subject to the reasonable approval of Acusphere. In such
event, the right of any Holder to include Registrable Securities in such
registration shall be conditioned upon such Holder's participation in such
underwriting and the inclusion of such Holder's Registrable Securities in the
underwriting to the extent provided herein. All Holders proposing to distribute
their securities through such underwriting shall (together with Acusphere as
provided in subsection 2.5(e)) enter into an underwriting agreement in customary
form with the underwriter or underwriters selected for such underwriting.
Notwithstanding any other provision of this Section 2.2, if the underwriter
advises the Initiating Holders in writing that marketing factors require a
limitation of the number of shares to be underwritten, then the Initiating
Holders shall so advise all Holders of Registrable Securities which would
otherwise be underwritten pursuant hereto, and the number of shares of
Registrable Securities that may be included in the underwriting shall be
allocated among all Holders thereof, including the Initiating Holders, in
proportion (as nearly as practicable) to the amount of Registrable Securities of
Acusphere owned by each Holder; provided, however, that the number of shares of
Registrable Securities to be included in such underwriting shall not be reduced
unless all other securities are first entirely excluded from the underwriting.

            (c) Notwithstanding the foregoing, if Acusphere shall furnish to the
Holders requesting registration pursuant to this Section 2.2, a certificate
signed by the Chief Executive Officer of Acusphere stating that in the good
faith judgment of the Board of Directors of Acusphere, it would be seriously
detrimental to Acusphere and its stockholders for a registration statement to be
filed and it is, therefore, essential to defer the filing of such registration
statement, Acusphere shall have the right to defer taking action with respect to
such filing for a period of not more than one hundred twenty (120) days after
receipt of the request of the
<PAGE>
                                      -6-

Initiating Holders; provided, however, that Acusphere may not utilize this right
more than once in any twelve-month period.

            (d) Acusphere shall not be obligated to effect, or to take any
action to effect, any registration pursuant to this Section 2.2 after Acusphere
has effected four (4) registrations on Form S-1 or its then equivalent pursuant
to this Section 2.2 and such registration statements have been declared or
ordered effective and the sales of Registrable Securities under such
registration statements have closed; provided, however, that each such
obligation shall be deemed to be satisfied only when a registration statement
covering at least sixty percent (60%) or more of the Registrable Securities held
by the Holders initiating such registration and requested to be included in such
registration statement has been declared or ordered effective and the sale of
such Registrable Securities under such registration statements have closed; and
further provided, however, that in any event (i) the Holders of at least forty
percent (40%) of the Registrable Securities shall be entitled to at least one
demand registration pursuant to Section 2.2(a) hereto, and (ii) the Holders of
at least a majority of the Series F-1 Stock, Series F-2 Stock, Series I-1 Stock,
Series I-2 Stock, Series J Stock, Series J-1 Stock, Series J-2 Stock and Series
J-3 Stock (including, for such purposes, any shares of Common Stock issued upon
conversion thereof) shall be entitled to at least one demand registration
pursuant to Section 2.2(a) hereto.

            (e) No incidental right under this Section 2.2 shall be construed to
limit any registration required under Section 2.3 or Section 2.4 herein.

      2.3 "Piggy-Back" Registration.

            (a) If (but without any obligation to do so) Acusphere proposes to
register (including for this purpose a registration effected by Acusphere for
stockholders other than the Holders) any of its stock or other securities under
the Act in connection with the public offering of such securities solely for
cash (other than a registration relating solely to the sale of securities to
participants in a stock plan, a registration on any form which does not include
substantially the same information as would be required to be included in a
registration statement covering the sale of the Registrable Securities, a
registration in which the only Common Stock being registered is Common Stock
issuable upon conversion of debt securities which are also being registered, or
a registration relating to the sale of securities in connection with a
transaction covered by Rule 145 promulgated under the Act), Acusphere shall, at
such time, promptly give each Holder written notice of such registration. Upon
the written request of each Holder given within twenty (20) days after mailing
of such notice by Acusphere in accordance with Section 4.5, Acusphere shall,
subject to the provisions of subsection 2.3(b), cause to be registered under the
Act all of the Registrable Securities that each such Holder has requested to be
registered.

            (b) Underwriting Requirements. In connection with any offering
involving an underwriting of shares of Acusphere's capital stock, Acusphere
shall not be required under this Section 2.3 to include any of the Holders'
securities in such underwriting unless such Holders accept the terms of the
underwriting as agreed upon between Acusphere and the underwriters selected by
it (or by other persons entitled to select the underwriters pursuant to the
terms of this Agreement), and then only in such quantity as the underwriters
determine in their sole discretion will not jeopardize the success of the
offering by Acusphere. If the total amount of securities,
<PAGE>
                                      -7-

including Registrable Securities, requested by stockholders to be included in
such offering exceeds the amount of securities to be sold (other than by
Acusphere) that the underwriters determine in their sole discretion is
compatible with the success of the offering, then Acusphere shall be required to
include in the offering only that number of such securities, including
Registrable Securities, which the underwriters determine in their sole
discretion will not jeopardize the success of the offering; provided, however,
there shall first be excluded from such registration statement all shares of
Common Stock sought to be included therein by (i) any director, consultant,
officer or employee of Acusphere or any subsidiary of Acusphere (except those
joining such registration pursuant to the Stock Repurchase and Registration
Agreement, dated April 30, 1996, among Acusphere, Robert S. Langer, Harry R.
Allcock and Sherri C. Oberg, the "Founders' Registration Agreement"), and (ii)
stockholders exercising any contractual or incidental registration rights
subordinate and junior to the rights of the Holders of Registrable Securities.
Thereafter, the shares of Common Stock sought to be included in such
registration statement pursuant to this Agreement and, if applicable, the
Founders' Registration Agreement, shall be excluded from such registration
statement in proportion (as nearly as practicable) to the amount of such shares
owned by the holders of such shares. If after such shares are excluded, the
underwriters shall determine in their sole discretion that the number of
securities which remain to be included in the offering exceeds the amount of
securities to be sold that the underwriters determine is compatible with the
success of the offering, then the Common Stock to be included, if any, shall be
apportioned pro rata among the holders of Common Stock providing notice of their
desire to participate in the offering pursuant to this Agreement and the
Founders' Registration Agreement (the "Selling Stockholders") according to the
total amount of securities entitled to be included therein owned by each Selling
Stockholder, or in such other proportions as shall mutually be agreed to by such
Selling Stockholders. Notwithstanding the foregoing, in no event shall the
amount of securities of the Selling Stockholders included in the offering be
reduced below thirty percent (30%) of the total amount of securities included in
such offering, unless such offering is the initial public offering of
Acusphere's securities in which case the Selling Stockholders may be excluded if
the underwriters make the determination described above and no other
stockholder's securities are included. For purposes of the preceding sentence
concerning apportionment, for any Selling Stockholder which is a partnership or
corporation, the partners, retired partners and stockholders of such Selling
Stockholder, or the estates and family members of any such partners and retired
partners and any trusts for the benefit of any of the foregoing persons shall be
deemed to be a single "Selling Stockholder", and any pro-rata reduction with
respect to such "selling holder" shall be based upon the aggregate amount of
shares carrying registration rights owned by all entities and individuals
included in such "Selling Stockholder", as defined in this sentence.

            (c) No incidental right under this Section 2.3 shall be construed to
limit any registration required under Section 2.2 or Section 2.4 herein.

      2.4 Form S-3 Registration. In case Acusphere shall receive from any Holder
or Holders of Registrable Securities a written request or requests that
Acusphere effect a registration on Form S-3 and any related qualification or
compliance with respect to all or a part of the Registrable Securities owned by
such Holder or Holders, Acusphere will:
<PAGE>
                                      -8-

            (a) promptly give written notice of the proposed registration, and
any related qualification or compliance, to all other Holders; and

            (b) as soon as practicable, effect such registration and all such
qualifications and compliances as may be so requested and as would permit or
facilitate the sale and distribution of all or such portion of such Holder's or
Holders' Registrable Securities as are specified in such request, together with
all or such portion of the Registrable Securities of any other Holder or Holders
joining in such request as are specified in a written request given within
fifteen (15) days after receipt of such written notice from Acusphere; provided,
however, that Acusphere shall not be obligated to effect any such registration,
qualification or compliance, pursuant to this Section 2.4: (i) if Form S-3 is
not available for such offering by the Holders; (ii) if the Holders, together
with the holders of any other securities of Acusphere entitled to inclusion in
such registration, propose to sell Registrable Securities and such other
securities (if any) at an aggregate price to the public of less than $3,000,000;
or (iii) if Acusphere shall furnish to the Holders a certificate signed by the
Chief Executive Officer of Acusphere stating that in the good faith judgment of
the Board of Directors of Acusphere, it would be seriously detrimental to
Acusphere and its stockholders for such Form S-3 registration to be effected at
such time, in which event Acusphere shall have the right to defer the filing of
the Form S-3 registration statement for a period of not more than sixty (60)
days after receipt of the request of the Holder or Holders under this Section
2.4; provided, however, that Acusphere shall not utilize this right more than
once in any twelve (12) month period.

            (c) Subject to the foregoing, Acusphere shall file a registration
statement covering the Registrable Securities and other securities so requested
to be registered as soon as practicable after receipt of the request or requests
of the Holders. At the election of a majority in interest of the Holders of
Registrable Securities to be included in such registration statement, such
registration may be firmly underwritten by a managing underwriter selected by
such Holders, and, if so elected, be subject to the requirements and provisions
of Section 2.2(b). Registrations effected pursuant to this Section 2.4 shall not
be counted as demands for registration or registrations effected pursuant to
Sections 2.2 or 2.3, respectively.

      2.5 Obligations of Acusphere. Whenever required under this Section 2 to
effect the registration of any Registrable Securities, Acusphere shall, as
expeditiously as reasonably possible:

            (a) Prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use its best efforts to cause such
registration statement to become effective, and, upon the request of the Holders
of a majority of the Registrable Securities registered thereunder, keep such
registration statement effective for a period of up to one hundred twenty (120)
days or until the distribution contemplated in the Registration Statement has
been completed; provided, however, that (i) such 120-day period shall be
extended for a period of time equal to the period the Holder refrains from
selling any securities included in such registration at the request of an
underwriter of Common Stock (or other securities) of Acusphere; and (ii) in the
case of any registration of Registrable Securities on Form S-3 which are
intended to be offered on a continuous or delayed basis, such 120-day period
shall be extended, if necessary, to keep the registration statement effective
until all such Registrable Securities are sold, provided that
<PAGE>
                                      -9-

Rule 415, or any successor rule under the Act, permits an offering on a
continuous or delayed basis, and provided further that applicable rules under
the Act governing the obligation to file a post-effective amendment permit, in
lieu of filing a post-effective amendment which (A) includes any prospectus
required by Section 10(a)(3) of the Act or (B) reflects facts or events
representing a material or fundamental change in the information set forth in
the registration statement, the incorporation by reference of information
required to be included in (A) and (B) above to be contained in periodic reports
filed pursuant to Section 13 or 15(d) of the 1934 Act in the registration
statement.

            (b) Prepare and file with the SEC such amendments and supplements to
such registration statement and the prospectus used in connection with such
registration statement as may be necessary to comply with the provisions of the
Act with respect to the disposition of all securities covered by such
registration statement.

            (c) Furnish to the Holders such numbers of copies of a prospectus,
including a preliminary prospectus, in conformity with the requirements of the
Act, and such other documents as they may reasonably request in order to
facilitate the disposition of Registrable Securities owned by them.

            (d) Use its best efforts to register and qualify the securities
covered by such registration statement under such other securities or Blue Sky
laws of such jurisdictions as shall be reasonably requested by the Holders;
provided that Acusphere shall not be required in connection therewith or as a
condition thereto to qualify to do business or to file a general consent to
service of process in any such states or jurisdictions, unless Acusphere is
already subject to service in such jurisdiction and except as may be required by
the Act.

            (e) In the event of any underwritten public offering, enter into and
perform its obligations under an underwriting agreement, in usual and customary
form, with the managing underwriter of such offering. Each Holder participating
in such underwriting shall also enter into and perform its obligations under
such an agreement.

            (f) Notify each Holder of Registrable Securities covered by such
registration statement at any time when a prospectus relating thereto is
required to be delivered under the Act as a result of the happening of any event
as a result of which the prospectus included in such registration statement, as
then in effect, includes an untrue statement of a material fact or omits to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading in the light of the circumstances then
existing.

            (g) Cause all such Registrable Securities registered pursuant
hereunder to be listed on each securities exchange on which similar securities
issued by Acusphere are then listed.

            (h) Provide a transfer agent and registrar for all Registrable
Securities registered pursuant hereunder and a CUSIP number for all such
Registrable Securities, in each case not later than the effective date of such
registration.
<PAGE>
                                      -10-

            (i) Furnish, at the request of any Holder requesting registration of
Registrable Securities pursuant to this Section 2, on the date that such
Registrable Securities are delivered to the underwriters for sale in connection
with a registration pursuant to this Section 2, if such securities are being
sold through underwriters, or, if such securities are not being sold through
underwriters, on the date that the registration statement with respect to such
securities becomes effective, (i) an opinion, dated such date, of the counsel
representing Acusphere for the purposes of such registration, in form and
substance as is customarily given to underwriters in an underwritten public
offering, addressed to the underwriters, if any, and to the Holders requesting
registration of Registrable Securities and (ii) a letter dated such date, from
the independent certified public accountants of Acusphere, in form and substance
as is customarily given by independent certified public accountants to
underwriters in an underwritten public offering, addressed to the underwriters,
if any, and to the Holders requesting registration of Registrable Securities.

            (j) Make available for inspection by each seller of Registrable
Securities, any underwriter participating in any distribution pursuant to such
registration statement, and any attorney, accountant or other agent retained by
such seller or underwriter, all financial and other records, pertinent corporate
documents and properties of Acusphere, and cause Acusphere's officers, directors
and employees to participate in "road show" meetings with potential investors
and to supply all information reasonably requested by any such seller,
underwriter, attorney, accountant or agent in connection with such registration
statement.

      2.6 Furnish Information. It shall be a condition precedent to the
obligations of Acusphere to take any action pursuant to this Section 2 with
respect to the Registrable Securities of any selling Holder that such Holder
shall furnish to Acusphere such information regarding itself, the Registrable
Securities held by it, and the intended method of disposition of such securities
as shall be required to effect the registration of such Holder's Registrable
Securities.

      2.7 Expenses of Demand and S-3 Registrations. All expenses other than
underwriting discounts and commissions incurred in connection with
registrations, filings or qualifications pursuant to Sections 2.2 and 2.4,
including (without limitation) all registration, filing and qualification fees,
printers' and accounting fees, fees and disbursements of counsel for Acusphere
and transfer taxes, if any, and the reasonable fees and disbursements of one
counsel for the selling Holders shall be borne by Acusphere; provided, however,
that Acusphere shall not be required to pay for any expenses of any registration
proceeding begun pursuant to Section 2.2 or 2.4 if the registration request is
subsequently withdrawn at the request of the Holders of a majority of the
Registrable Securities to be registered (in which case all Holders participating
in the registration shall bear such expenses), unless, in the case of a
registration pursuant to Section 2.2, the Holders of a majority of the
Registrable Securities agree to forfeit their right to one demand registration
pursuant to Section 2.2; provided further, however, that if at the time of such
withdrawal, the Holders have learned of a material adverse change in the
condition, business, or prospects of Acusphere from that known to the Holders at
the time of their request and have withdrawn the request with reasonable
promptness following disclosure by Acusphere of such material adverse change,
then the Holders shall not be required to pay any of such expenses and shall
retain their rights under Sections 2.2 and 2.4.
<PAGE>
                                      -11-

      2.8 Expenses of "Piggy-Back" Registration. Acusphere shall bear and pay
all expenses incurred in connection with any registration, filing or
qualification of Registrable Securities with respect to the registrations
pursuant to Section 2.3 for each Holder, including (without limitation) all
registration, filing, and qualification fees, printers' and accounting fees
relating or apportionable thereto and transfer taxes, if any, and the fees and
disbursements of one counsel for the selling Holders selected by them, but
excluding underwriting discounts and commissions relating to the Registrable
Securities.

      2.9 Delay of Registration. No Holder shall have any right to obtain or
seek an injunction restraining or otherwise delaying any such registration as
the result of any controversy that might arise with respect to the
interpretation or implementation of this Section 2.

      2.10 Indemnification. In the event any Registrable Securities are included
in a registration statement under this Section 2:

            (a) To the extent permitted by law, Acusphere will indemnify and
hold harmless each Holder, any underwriter (as defined in the Act) for such
Holder and each person, if any, who controls such Holder or underwriter within
the meaning of the Act or the 1934 Act, against any losses, claims, damages, or
liabilities (joint or several) to which they may become subject under the Act,
the 1934 Act or other federal or state law, insofar as such losses, claims,
damages, or liabilities (or actions in respect thereof) arise out of or are
based upon any of the following statements, omissions or violations
(collectively a "Violation"): (i) any untrue statement or alleged untrue
statement of a material fact contained in such registration statement, including
any preliminary prospectus or final prospectus contained therein or any
amendments or supplements thereto, (ii) the 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 violation or alleged
violation by Acusphere of the Act, the 1934 Act, any state securities law or any
rule or regulation promulgated under the Act, the 1934 Act or any state
securities law; and Acusphere will pay to each such Holder, underwriter or
controlling person, as incurred, any legal or other expenses reasonably incurred
by them in connection with investigating or defending any such loss, claim,
damage, liability, or action; provided, however, that the indemnity agreement
contained in this subsection 2.10(a) shall not apply to amounts paid in
settlement of any such loss, claim, damage, liability, or action if such
settlement is effected without the consent of Acusphere (which consent shall not
be unreasonably withheld), nor shall Acusphere be liable in any such case for
any such loss, claim, damage, liability, or action to the extent that it arises
out of or is based upon a Violation which occurs in reliance upon and in
conformity with written information furnished expressly for use in connection
with such registration by any such Holder, underwriter or controlling person.

            (b) To the extent permitted by law, each selling Holder, severally
but not jointly, will indemnify and hold harmless Acusphere, each of its
directors, each of its officers who has signed the registration statement, each
person, if any, who controls Acusphere within the meaning of the Act, any
underwriter, any other Holder selling securities in such registration statement
and any controlling person of any such underwriter or other Holder, against any
losses, claims, damages, or
<PAGE>
                                      -12-

liabilities, to which any of the foregoing persons may become subject, under the
Act, the 1934 Act or other federal or state law, insofar as such losses, claims,
damages, or liabilities (or actions in respect thereto) arise out of or are
based upon any Violation, in each case to the extent (and only to the extent)
that such Violation occurs in reliance upon and in conformity with written
information furnished by such Holder expressly for use in connection with such
registration; and each such Holder will pay, as incurred, any legal or other
expenses reasonably incurred by any person intended to be indemnified pursuant
to this subsection 2.10(b), in connection with investigating or defending any
such loss, claim, damage, liability, or action; provided, however, that the
indemnity agreement contained in this subsection 2.10(b) shall not apply to
amounts paid in settlement of any such loss, claim, damage, liability or action
if such settlement is effected without the consent of the Holder, which consent
shall not be unreasonably withheld; provided, that, in no event shall any
indemnity under this subsection 2.10(b) exceed the net proceeds from the
offering received by such Holder.

            (c) Promptly after receipt by an indemnified party under this
Section 2.10 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this Section 2.10, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; provided, however, that an indemnified party
(together with all other indemnified parties which may be represented without
conflict by one counsel) shall have the right to retain one separate counsel,
with the fees and expenses to be paid by the indemnifying party, if the
indemnified party shall have reasonably concluded that there may be reasonable
defenses available to it which are different from or additional to those
available to the indemnifying party or if the interests of the indemnified party
reasonably may be deemed to conflict with the interests of the indemnifying
party. The failure to deliver written notice to the indemnifying party within a
reasonable time of the commencement of any such action, if prejudicial to its
ability to defend such action, shall relieve such indemnifying party of any
liability to the indemnified party under this Section 2.10, but the omission so
to deliver written notice to the indemnifying party will not relieve it of any
liability that it may have to any indemnified party otherwise than under this
Section 2.10.

            (d) If the indemnification provided for in this Section 2.10 is held
by a court of competent jurisdiction to be unavailable to an indemnified party
with respect to any loss, liability, claim, damage, or expense referred to
therein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable by such
indemnified party as a result of such loss, liability, claim, damage, or expense
in such proportion as is appropriate to reflect the relative fault of the
indemnifying party on the one hand and of the indemnified party on the other in
connection with the statements or omissions that resulted in such loss,
liability, claim, damage, or expense as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission to state a material fact relates to information supplied by the
indemnifying party or by the indemnified party and the parties' relative intent,
knowledge, access to information, and opportunity to correct or prevent such
statement or omission; provided, that, in the case of contribution by a selling
Holder, in no event shall any contribution under this subsection 2.10(d) exceed
the net proceeds from the offering received by such contributing party.


<PAGE>




                                      -13-


            (e) Notwithstanding the foregoing, to the extent that the provisions
on indemnification and contribution contained in the underwriting agreement
entered into in connection with the underwritten public offering are in conflict
with the foregoing provisions, the provisions in the underwriting agreement
shall control.

            (f) The obligations of Acusphere and Holders under this Section 2.10
shall survive the completion of any offering of Registrable Securities in a
registration statement under this Section 2, and otherwise.

      2.11 Reports Under Securities Exchange Act of 1934. With a view to making
available to the Holders the benefits of Rule 144 promulgated under the Act and
any other rule or regulation of the SEC that may at any time permit a Holder to
sell securities of Acusphere to the public without registration or pursuant to a
registration on Form S-3, Acusphere agrees to use its best efforts to:

            (a) make and keep public information available, as those terms are
understood and defined in SEC Rule 144, at all times after ninety (90) days
after the effective date of the first registration statement filed by Acusphere
for the offering of its securities to the general public;

            (b) take such action, including the voluntary registration of its
Common Stock under Section 12 of the 1934 Act, as is necessary to enable the
Holders to utilize Form S-3 for the sale of their Registrable Securities, such
action to be taken as soon as practicable after the end of the fiscal year in
which the first registration statement filed by Acusphere for the offering of
its securities to the general public is declared effective;

            (c) file with the SEC in a timely manner all reports and other
documents required of Acusphere under the Act and the 1934 Act; and

            (d) furnish to any Holder, so long as the Holder owns any
Registrable Securities, forthwith upon request (i) a written statement by
Acusphere that it has complied with the reporting requirements of SEC Rule 144
(at any time after ninety (90) days after the effective date of the first
registration statement filed by Acusphere), the Act and the 1934 Act (at any
time after it has become subject to such reporting requirements), or that it
qualifies as a registrant whose securities may be resold pursuant to Form S-3
(at any time after it so qualifies), (ii) a copy of the most recent annual or
quarterly report of Acusphere and such other reports and documents so filed by
Acusphere, and (iii) such other information as may be reasonably requested in
availing any Holder of any rule or regulation of the SEC which permits the
selling of any such securities without registration or pursuant to such form.

      2.12 Assignment of Registration Rights. The rights to cause Acusphere to
register Registrable Securities pursuant to this Section 2 may be assigned (but
only with all related obligations) by a Holder only to a transferee or assignee
of such securities who, after such assignment or transfer, holds at least 75,000
shares of Registrable Securities (subject to appropriate adjustment for stock
splits, stock dividends, combinations and other recapitalizations), provided:
(a) Acusphere is, within a reasonable time after such transfer, furnished with
written notice of the name and address of such transferee or assignee and the
<PAGE>
                                      -14-


securities with respect to which such registration rights are being assigned;
(b) such transferee or assignee agrees in writing to be bound by and subject to
the terms and conditions of this Agreement, including without limitation the
provisions of Section 2.14 below; and (c) such assignment shall be effective
only if immediately following such transfer the further disposition of such
securities by the transferee or assignee is restricted under the Act.

      2.13 Limitations on Subsequent Registration Rights. From and after the
date of this Agreement, Acusphere shall not, without the prior written consent
of the Holders of a majority of the Registrable Securities then outstanding,
enter into any agreement with any holder or prospective holder of any securities
of Acusphere which would allow such holder or prospective holder (a) to include
such securities in any registration filed under Section 2.2 or described in
Section 2.3 hereof, unless under the terms of such agreement, such holder or
prospective holder may include such securities in any such registration only to
the extent that the inclusion of such securities will not reduce the amount of
the Registrable Securities of the Holders which are to be included or (b) to
make a demand registration which could result in such registration statement
being declared effective prior to the earlier of either of the dates set forth
in subsection 2.2(a) or within one hundred twenty (120) days of the effective
date of any registration effected pursuant to Section 2.2 or 2.4.

      2.14 "Market Stand-Off" Agreement. Each Investor hereby agrees that,
during the period of duration (not to exceed one hundred eighty (180) days)
specified by Acusphere and an underwriter of Common Stock or other securities of
Acusphere, following the effective date of a registration statement of Acusphere
filed under the Act, such Investor shall not, to the extent requested by
Acusphere and such underwriter, directly or indirectly sell, offer to sell,
contract to sell (including, without limitation, any short sale), grant any
option to purchase or otherwise transfer or dispose of (other than to donees who
agree to be similarly bound) any securities of Acusphere held by it at any time
during such period except Common Stock included in such registration; provided,
however, that:

            (a) such agreement shall be applicable only with respect to a
      registration statement covering Acusphere's initial public offering of
      securities; and
            (b) all officers and directors of Acusphere and all other persons
      with registration rights (whether or not pursuant to this Agreement) enter
      into similar agreements.

            In order to enforce the foregoing covenant, Acusphere may impose
stop-transfer instructions with respect to the Registrable Securities of an
Investor (and the shares or securities of every other person subject to the
foregoing restriction) until the end of such period.

            Notwithstanding the foregoing, the obligations described in this
Section 2.14 shall not apply to a registration relating solely to employee
benefit plans on Form S-1 or Form S-8 or similar forms which may be promulgated
in the future, or a registration relating solely to a SEC Rule 145 transaction
on Form S-4 or similar forms which may be promulgated in the future.
<PAGE>
                                      -15-


      2.15 Termination of Registration Rights. No Holder shall be entitled to
exercise any right provided for in this Section 2 if there is then an active
public trading market for the Common Stock, such Holder may sell all of such
Holder's Registrable Securities under Rule 144 promulgated under the Act in a
three-month period and such Holder's Registrable Securities represent less than
3% of Acusphere's outstanding shares of capital stock. No holder shall be
entitled to exercise any right provided for in Section 2.2 after five (5) years
following the consummation of the sale of securities pursuant to a registration
statement filed by Acusphere under the Act in connection with the initial firm
commitment underwritten offering of its securities to the general public.

3. Covenants of Acusphere.


      3.1 Delivery of Financial Statements. Acusphere shall deliver to each
Significant Investor (as defined in Section 3.2(b) hereof), as long as such
Investor holds shares of Preferred Stock (or any shares of Common Stock issuable
upon conversion thereof):

            (a) as soon as practicable, but in any event within ninety (90) days
after the end of each fiscal year of Acusphere, an income statement for such
fiscal year, a balance sheet of Acusphere and statement of stockholders' equity
as of the end of such year, and a schedule as to the sources and applications of
funds for such year, such year-end financial reports to be in reasonable detail,
prepared in accordance with generally accepted accounting principles ("GAAP"),
and audited and certified by independent public accountants of nationally
recognized standing selected by Acusphere;

            (b) as soon as practicable, but in any event within forty-five (45)
days after the end of each of the first three (3) quarters of each fiscal year
of Acusphere, an unaudited profit or loss statement and schedule as to the
sources and application of funds for such fiscal quarter and an unaudited
balance sheet as of the end of such fiscal quarter;

            (c) within thirty (30) days of the end of each month, an unaudited
income statement and schedule as to the sources and application of funds and
balance sheet for and as of the end of such month, in reasonable detail;

            (d) as soon as practicable, but in any event forty-five (45) days
prior to the end of each fiscal year, a budget and business plan for the next
fiscal year, prepared on a monthly basis, including balance sheets and sources
and applications of funds statements for such months and, as soon as prepared,
any other budgets or revised budgets prepared by Acusphere;

            (e) with respect to the financial statements called for in
subsections (a), (b) and (c) of this Section 3.1, an instrument executed by the
Chief Financial Officer or Chief Executive Officer of Acusphere and certifying
that such financials were prepared in accordance with GAAP consistently applied
with prior practice for earlier periods (with the exception of footnotes that
may be required by GAAP) and fairly present the financial condition of Acusphere
and its results of operation for the period specified, subject to year-end audit
adjustment; and
<PAGE>
                                      -16-


            (f) such other information relating to the financial condition,
business, prospects or corporate affairs of Acusphere as such Significant
Investor or any assignee of such Significant Investor may from time to time
request, provided, however, that Acusphere shall not be obligated under this
subsection (f) or any other subsection of Section 3.1 to provide information
which it deems in good faith to be a trade secret or similar confidential
information.

      3.2 Inspection and Observation.


            (a) Acusphere shall permit each Significant Investor (as defined in
Section 3.2(b) hereof), at such Significant Investor's expense, to visit and
inspect Acusphere's properties, to examine its books of account and records and
to discuss Acusphere's affairs, finances and accounts with its officers, all at
such reasonable times as may be requested by such Significant Investor;
provided, however, that Acusphere shall not be obligated pursuant to this
Section 3.2 to provide access to any information which its Board of Directors
reasonably considers to be a trade secret or similar confidential information.

            (b) Acusphere will permit any Significant Investor (as hereinafter
defined), any authorized representative thereof or any Investor holding shares
of Series E-1 Stock and/or Series E-2 Stock and at least five percent (5%) of
the Common Stock on a fully-diluted basis, assuming the conversion or exercise,
as the case may be, of all outstanding securities or rights to acquire shares of
Acusphere's Common Stock (each, a "Series E Five-Percent Holder"), to attend all
meetings of the Board of Directors of Acusphere in a nonvoting observer capacity
and shall, upon the written request of such Significant Investor or Series E
Five-Percent Holder, provide such Significant Investor or Series E Five-Percent
Holder with such notice and other information with respect to such meetings as
are delivered to the directors of Acusphere; provided, however, that it is
hereby acknowledged and agreed that only a single representative of each group
consisting of a Significant Investor and its affiliates will be entitled to
attend any such meeting pursuant to this Section 3.2(b). Upon the written
request of any such Significant Investor or Series E Five-Percent Holder,
Acusphere shall notify such Significant Investor or Series E Five-Percent
Holder, within ten (10) days thereof, of the taking of any action by the Board
of Directors of Acusphere in lieu of a meeting thereof. As used in this Section
3, the term "Significant Investor" shall mean any Investor other than Alexandria
who, together with its affiliates, holds at least 200,000 shares of Preferred
Stock, including Common Stock issuable upon the conversion of Preferred Stock,
(as presently constituted and subject to subsequent adjustment for stock splits,
stock dividends, reverse stock splits, recapitalizations and the like).

      3.3 Termination of Information, Inspection and Observation Covenants,
Assignment. The covenants set forth in Section 3.1 and Section 3.2 shall
terminate as to each Investor and be of no further force or effect when the sale
of securities pursuant to a registration statement filed by Acusphere under the
Act in connection with the firm commitment underwritten offering of its
securities to the general public is consummated or when Acusphere first becomes
subject to the periodic reporting requirements of Sections 12(g) or 15(d) of the
1934 Act, whichever event shall first occur. The rights to receive and access
information relating to Acusphere pursuant to Sections 3.1 and 3.2 may be
assigned (but only with all related obligations) by an Investor to a transferee
or assignee of such securities who, after such assignment or transfer, holds at
least 75,000 shares of Registrable Securities (subject to appropriate adjustment
for stock splits, stock
<PAGE>
                                      -17-


dividends, combinations and other recapitalizations), provided: (a) Acusphere
is, within a reasonable time after such transfer, furnished with written notice
of the name and address of such transferee or assignee and the securities with
respect to which such information rights are being assigned; and (b) such
transferee or assignee agrees in writing to be bound by and subject to the terms
and conditions of this Agreement.

      3.4 Right of First Refusal. Subject to the terms and conditions specified
in this Section 3.4, Acusphere hereby grants to each of the Investors a right of
first refusal with respect to future sales by Acusphere of its Shares and
Venture Capital Shares (each, as hereinafter defined). For purposes of this
Section 3.4 and Section 4 only, the term "Investor" shall include Oberg and any
general partners and affiliates of an Investor. Each Investor shall be entitled
to apportion the right of first refusal hereby granted it among itself and its
partners and affiliates in such proportions as it deems appropriate. For
purposes hereof, an "affiliate" of any Investor is an entity or a person that
directly or indirectly controls, is controlled by, or is under common control
with such Investor. For purposes hereof, (i) all notice provisions shall be
deemed satisfied with respect to any general partner or affiliate of an Investor
if such notice is deemed satisfied with respect to such Investor, (ii) all
waiver and amendment provisions shall be deemed satisfied with respect to any
general partner or affiliate of an Investor if such waiver or amendment is
deemed satisfied with respect to such Investor, and (iii) in no event shall any
Investor, its general partners and its affiliates, together as a group, have a
right pursuant to this Section 3.4 to purchase in the aggregate more than such
Investor's pro rata portion of the Shares and Venture Capital Shares, if any,
offered hereby.

            Each time Acusphere proposes to offer any shares of, or securities
convertible into or exercisable for any shares of, any class of its capital
stock (the "Shares"), Acusphere shall first make an offering of such Shares to
the Investors in accordance with the following provisions:

            (a) Acusphere shall deliver a notice by certified mail (the
"Notice") to each Investor other than Alexandria (the "Remaining Investors")
stating (i) its bona fide intention to offer such Shares, (ii) the number of
such Shares to be offered, and (iii) the price and terms, if any, upon which it
proposes to offer such Shares. If the Shares being offered by Acusphere are
offered in connection with a Venture Capital Financing (as defined in paragraph
(c) below), the Notice shall be delivered to the Remaining Investors
simultaneously with the Alexandria Notice (as defined below) delivered to
Alexandria.

            (b) Within thirty (30) calendar days after receipt of the Notice,
each Remaining Investor may elect to purchase or obtain, at the price and on the
terms specified in the Notice (provided, that, in the event any such
consideration is non-cash consideration, at the election of such Remaining
Investor, such Remaining Investor may pay cash equal to the fair market value of
such non-cash consideration, and provided, that, the closing of any such
purchase may be extended to any such Remaining Investor for purposes of
obtaining necessary governmental approvals), some or all of such Remaining
Investor's pro rata portion of the Shares offered by Acusphere in the Notice.
Each Remaining Investor's pro rata portion of such Shares shall be equal to a
fraction of such Shares, the numerator of which is the number of shares of
Registrable Securities or Common Stock as the case may be, then held by such
Remaining Investor and the denominator of which shall be equal to the sum of the
total number of shares of
<PAGE>
                                      -18-


Registrable Securities and Common Stock then held by all Remaining Investors. If
any Remaining Investor does not elect to exercise its or his right to purchase
its or his total pro rata portion of the Shares, Acusphere shall promptly give
notice of such failure to the other Remaining Investors who did elect to
purchase (the "Participants"). Each Remaining Investor shall have a right of
over-allotment such that if any Remaining Investor fails to exercise its or his
right to purchase its or his total pro rata portion of the Shares, the
Participants may purchase such portion, by giving written notice to Acusphere
with five (5) days from the date that Acusphere provides written notice to the
other Participants of the number of Shares with respect to which such
non-purchasing Remaining Investor has failed to exercise its rights hereunder.

            (c) Notwithstanding the foregoing rights contained in clause (b) of
this Section 3.4, if such Shares are offered in connection with a private
placement pursuant to a Venture Capital Financing (as defined below) ("Venture
Capital Shares"), Acusphere shall first deliver a notice by certified mail (the
"Alexandria Notice") to Alexandria stating (i) its bona fide intention to offer
such Venture Capital Shares, (ii) the number of Venture Capital Shares to be
offered, and (iii) the price and terms, if any, upon which it proposes to offer
such Venture Capital Shares. Within thirty (30) calendar days after receipt of
the Alexandria Notice, Alexandria may elect to purchase or obtain, at the price
and on the terms specified in the Alexandria Notice (provided, that, in the
event any such consideration is non-cash consideration, at the election of
Alexandria, Alexandria may pay cash equal to the fair market value of such
non-cash consideration, and provided, that, the closing of any such purchase may
be extended to Alexandria for purposes of obtaining necessary governmental
approvals), such number of Venture Capital Shares as is equal the number of
Venture Capital Shares determined by dividing $250,000 by the per share price at
which the Venture Capital Shares are offered by Acusphere. The term "Venture
Capital Financing" as used herein shall refer to the sale by Acusphere of
Venture Capital Shares in a private placement to one or more institutional
investors resulting in aggregate gross proceeds to Acusphere of at least
$5,000,000, excluding any securities issued in connection with an acquisition
transaction. In the foregoing clauses (a) and (b) of this Section 3.4, the term
Shares shall exclude any such Shares which constitute Venture Capital Shares and
which are purchased by Alexandria pursuant to this clause (c).

            (d) If the Investors do not elect to purchase all of the Shares and
Venture Capital Shares, if applicable, referred to in the Notice and Alexandria
Notice, if applicable, Acusphere may, during the 90-day period following the
expiration of the periods provided in subsections 3.4(b) and (c) hereof
(including the additional five day period provided for over-allotments), offer
the remaining unsubscribed portion of such Shares and Venture Capital Shares, if
applicable, to any person or persons at a price not less than, and upon terms no
more favorable to the offeree than those specified in the Notice and Alexandria
Notice, if applicable. If Acusphere does not enter into an agreement for the
sale of the unsubscribed portion of such Shares and Venture Capital Shares,
within such period, or if such agreement is not consummated within thirty (30)
days of the execution thereof, the right provided hereunder shall be deemed to
be revived and such Shares and Venture Capital Shares, shall not be offered
unless first reoffered to the Investors in accordance herewith; provided,
however, that if during such 90-day period, Acusphere offers such Shares and
Venture
<PAGE>
                                      -19-


Capital Shares, if applicable, to any person or persons at a price and/or upon
terms more favorable to the offeree than those specified in the Notice and
Alexandria Notice, if applicable, then Acusphere must first reoffer the Shares
and Venture Capital Shares, if applicable, to the Investors on such favorable
terms and the procedure set forth above shall be followed, with the exception
that the 30-day periods set forth in subsections 3.4(a) and (c) shall be a
15-day period.

            (e) The rights of first refusal in this Section 3.4 shall not be
applicable (i) to the issuance or sale on or after March 30, 1994, of up to
6,591,977 shares of Common Stock (or options therefor) to Acusphere's employees,
consultants, or directors (adjusted to reflect subsequent stock dividends, stock
splits or recapitalizations affecting the number of outstanding shares of the
Common Stock) for the primary purpose of soliciting or retaining their services,
provided each such employee, consultant or director executes an agreement
providing for (A) vesting of shares (or the option to purchase shares) over at
least a four-year period, (B) market stand-off agreements similar to that set
forth in Section 2.14 of this Agreement, and (C) rights of first refusal on
transfers of such shares which shall first be a right of Acusphere, then of
Acusphere's assignees; provided, however, that any such requirements may be
waived by action of Acusphere's Board of Directors, provided that all
representatives of the Investors on the Board of Directors of Acusphere consent
to such waiver, (ii) to the issuance of securities in or after consummation of a
bona fide, firmly underwritten initial public offering of shares of Common
Stock, registered under the Act pursuant to a registration statement on Form S-1
or other available form, (iii) to the issuance of securities pursuant to the
conversion of convertible securities, (iv) to the issuance of 130,169 and 22,216
shares of Common Stock to the Massachusetts Institute of Technology ("MIT") on
or about March 30, 1995 and June 4, 1996, respectively, (v) to the issuance of
securities solely to prevent dilution as a result of any stock splits, stock
dividends, combinations, recapitalizations or issuances of securities below the
then applicable conversion price for the Preferred Stock, (vi) to the issuance
of securities (including options, warrants or convertible securities and the
securities issuable upon conversion or exercise of such options, warrants or
convertible securities) to lending or leasing institutions pursuant to debt or
lease financing arrangements approved by the Board of Directors, including all
of the directors then serving on the Board of Directors which were designated by
the holders of the Company's Preferred Stock in accordance with the Ninth
Amended and Restated Voting Agreement, dated as of the date hereof, as amended
and in effect from time to time (the "Voting Agreement"), not to exceed, in the
aggregate, five percent (5.0%) of the capital stock of Acusphere on a
fully-diluted basis (assuming the conversion or exercise of all outstanding
option, warrants or convertible securities), (vii) to the issuance of the Notes
and Warrants issued pursuant to the Note and Warrant Purchase Agreement
(including the shares of Acusphere capital stock, if any, issued or issuable
upon conversion, exercise or exchange thereof, and the Common Stock issued or
issuable upon conversion thereof), (viii) to the issuance of Series G
Paid-in-Kind Dividends pursuant to Section 1.8 Acusphere's Tenth Amended and
Restated Certificate of Incorporation, or (ix) to the issuance of the Alexandria
Warrant, the GATX Warrant, or the VLL Warrant, or the Alexandria Warrant Shares
or the Venture Lender Warrant Shares issued or issuable upon exercise thereof,
and the shares of Common Stock issued or issuable upon conversion thereof.

            (f) The right of first refusal set forth in this Section 3.4 may be
assigned (but only with all related obligations) by a Holder only to a
transferee or assignee of such securities who, after such assignment or
transfer, holds at least 75,000 shares of Registrable Securities (subject to
appropriate adjustment for stock splits, stock dividends, combinations and other
<PAGE>
                                      -20-


recapitalizations), provided: (a) Acusphere is, within a reasonable time after
such transfer, furnished with written notice of the name and address of such
transferee or assignee and the securities with respect to which such
registration rights are being assigned; (b) such transferee or assignee agrees
in writing to be bound by and subject to the terms and conditions of this
Agreement; and (c) such assignment shall be effective only if immediately
following such transfer the further disposition of such securities by the
transferee or assignee is restricted under the Act.

      3.5 Preparation of Audited Financial Statements. Acusphere shall undertake
to prepare and shall deliver to each Investor other than Alexandria, as soon as
practicable, its audited financial statements (balance sheet, profit and loss
statement, statement of stockholders' equity and statement of cash flows,
including notes thereon) as of December 31, 2002, and for each fiscal year
thereafter.

      3.6 Stock Purchase Agreements. All officers and employees of and
consultants to Acusphere who purchase shares of Acusphere's capital stock shall
be required to execute a stockholder agreement providing for (i) vesting of
shares (or option to purchase shares) over a four-year period, (ii) market
stand-off agreements similar to that set forth in Section 2.14 of this
Agreement, and (iii) rights of first refusal on transfers of such shares which
shall first be a right of Acusphere, then of Acusphere's assignees; provided
however, that any such requirements may be waived by action of Acusphere's Board
of Directors, provided that any representative of the Investors on the Board of
Directors of Acusphere consents to such waiver.

      3.7 Termination of Certain Covenants. The provisions set forth in Section
3.6 shall terminate and be of no further force or effect upon the consummation
of the sale of securities pursuant to a registration statement filed by
Acusphere under the Act in connection with the firm commitment underwritten
offering of its securities to the general public.

      3.8 Covenants Relating to SBA Matters. At such times as any SBIC Investor
holds any securities of Acusphere, Acusphere shall comply with the covenants set
forth in this Section 3.8.

            (a) Use of Proceeds. The proceeds from the prior issuances and sales
of preferred stock which the Company still retains (the "Proceeds") shall be
used by Acusphere for its growth, modernization or expansion. Acusphere shall
provide each SBIC Investor and the Small Business Administration (the "SBA")
reasonable access to Acusphere's books and records for the purpose of confirming
the use of Proceeds.

            (b) Business Activity. During the period ending on April 25, 2001,
Acusphere shall not change the nature of its business activity if such change
would render Acusphere ineligible as provided in 13 C.F.R. Section 107.720.

            (c) Compliance. So long as any SBIC Investor holds any securities of
Acusphere, Acusphere will at all times comply with the non-discrimination
requirements of 13 C.F.R. Parts 112, 113 and 117.
<PAGE>
                                      -21-


            (d) Information. Within 45 days after the end of each fiscal year
and at such other times as an SBIC Investor may reasonably request, Acusphere
shall deliver to such SBIC Investor a written assessment, in form and substance
satisfactory to such SBIC Investor, of the economic impact of such SBIC
Investor's investment specifying the full-time equivalent jobs created or
retained in connection with such investment, and the impact of the investment on
Acusphere's business in terms of profits and on taxes paid by Acusphere and its
employees. Upon request, Acusphere promptly (and in any event within 20 days of
such request) will furnish to each SBIC Investor all information reasonably
requested by such SBIC Investor in order for such SBIC Investor to comply with
the requirements of 13 C.F.R. Section 107.620 or to prepare and file SBA Form
468 and any other information requested or required by any governmental agency
asserting jurisdiction over such SBIC Investor. Acusphere shall afford to
representatives of the SBA reasonable access to the books, records and
properties of Acusphere and its subsidiaries. Any submission of any financial
information under Section 3.1, above, or this Section 3.8(d) shall include a
certificate of Acusphere's president, chief executive officer, treasurer or
chief financial officer. If a SBIC Investor requests confidential information
from Acusphere for the purpose of providing such information to the SBA, then,
if requested in writing by Acusphere at the time Acusphere provides such
information to such SBIC Investor, the SBIC Investor shall, to the extent
permitted by law, request that such confidential information be treated
confidentially by the SBA.

            (e) Number of Holders of Voting Securities. Acusphere shall notify
each SBIC Investor (i) on or prior to the taking of any action which increases
the number of record holders of Acusphere's voting securities from fewer than 50
to 50 or more, and (ii) of any other action or occurrence which increases the
number of record holders of Acusphere's voting securities from fewer than 50 to
50 or more, as soon as practicable after Acusphere becomes aware that such other
action or occurrence has occurred or is proposed to occur.

      3.9 Regulatory Compliance Cooperation. In the event that either Bank of
America Ventures or BancBoston Ventures determines that it has a Regulatory
Problem (as defined below), it shall have the right to transfer its Registrable
Securities without regard to any restrictions on transfer set forth in this
Agreement, the Note and Warrant Purchase Agreement, the Voting Agreement, or the
Third Amended and Restated Stock Restriction Agreement dated as of the date
hereof and in effect from time to time (the "Stock Restriction Agreement") and
Acusphere shall take all such actions as are reasonably requested by Bank of
America Ventures or BancBoston Ventures, as applicable, in order to (i)
effectuate and facilitate any transfer by it of any securities of Acusphere then
held by it to any person designated by Bank of America Ventures or BancBoston
Ventures, as applicable, provided that such person is reasonably acceptable to
Acusphere and agrees in writing to be bound by this Agreement, the Note and
Warrant Purchase Agreement, the Voting Agreement, and the Stock Restriction
Agreement, (ii) permit Bank of America Ventures or BancBoston Ventures (or any
of their affiliates), as applicable, to exchange all or any portion of any
voting security of Acusphere then held by it on a share-for-share basis for
shares of a nonvoting security of Acusphere, which nonvoting security shall be
identical in all respects to the voting security exchanged for it, except that
it shall be nonvoting and shall be convertible into a voting security on such
terms as are requested by it in light of regulatory considerations then
prevailing and (iii) amend this Agreement, as amended from time to time, to
effectuate and reflect the foregoing. The parties to this Agreement agree to
<PAGE>
                                      -22-


vote all of Acusphere's securities held by them in favor of such amendments and
actions. For purposes of this Agreement, a "Regulatory Problem" means any set of
facts or circumstances wherein it has been asserted by any governmental
regulatory agency that Bank of America Ventures or BancBoston Ventures is not
entitled to hold, or exercise any significant right with respect to securities
of Acusphere.

      3.10 Consideration of Section 1202(c). At such time or times, if any, that
Acusphere considers repurchasing any shares of its Preferred Stock, Acusphere
agrees to consider the effect that such repurchase would have on such stock's
qualification as "Qualified Small Business Stock," as defined in Section 1202(c)
of the Internal Revenue Code of 1986, as amended (the "Code"). Notwithstanding
the foregoing, Acusphere shall have no obligation to take, or refrain from
taking any action that would affect such stock's status as "Qualified Small
Business Stock." Acusphere will use reasonable efforts to comply with the
reporting and recordkeeping requirements of Section 1202 of the Code and the
regulations promulgated thereunder.

      3.11 Registration Rights. Acusphere shall not grant to any person or
entity registration rights which are on par with or senior to the registration
rights granted to the holders of the Registrable Securities, unless holders of a
majority of the then outstanding shares of Registrable Securities consent in
writing.

4. Miscellaneous.


      4.1 Successors and Assigns. Except as otherwise provided herein, the terms
and conditions of this Agreement shall inure to the benefit of and be binding
upon the respective successors and assigns of the parties (including transferees
of any shares of Registrable Securities). Nothing in this Agreement, express or
implied, is intended to confer upon any party other than the parties hereto or
their respective successors and assigns any rights, remedies, obligations, or
liabilities under or by reason of this Agreement, except as expressly provided
in this Agreement.

      4.2 Governing Law. This Agreement shall be governed by and construed under
the laws of the Commonwealth of Massachusetts as applied to agreements among
Massachusetts residents entered into and to be performed entirely within
Massachusetts.

      4.3 Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument. Any person who may, after
the date hereof, purchase Notes and Warrants pursuant to the Note and Warrant
Purchase Agreement shall become a party to this Agreement as an "Investor" and a
holder of "Notes," "Warrants," "Bridge Note Registrable Securities," "Bridge
Warrant Registrable Securities," and "Registrable Securities," as applicable for
all purposes hereunder, all upon execution of a counterpart to this Agreement
signed by such person and Acusphere.

      4.4 Titles and Subtitles. The titles and subtitles used in this Agreement
are used for convenience only and are not to be considered in construing or
interpreting this Agreement.

      4.5 Notices. Unless otherwise provided, any notice required or permitted
under this Agreement shall be given in writing and shall be deemed effectively
given (i) upon personal
<PAGE>
                                      -23-


delivery to the party to be notified, (ii) upon deposit with the United States
Post Office, by registered or certified mail, postage prepaid or (iii) one day
after deposit with a nationally recognized overnight courier, specifying next
day delivery, with written verification of receipt. All communications shall be
sent to the party to be notified at the address as set forth on the signature
pages hereof or the exhibits hereto or at such other address as such party may
designate by advance written notice to the other parties hereto; provided,
however, that registered or certified mail shall not be used to effectuate
delivery of any such notice, demand or other communication to any such parties
with principal places of business located outside the United States.

      4.6 Expenses. If any action at law or in equity is necessary to enforce or
interpret the terms of this Agreement, the prevailing party shall be entitled to
reasonable attorneys' fees, costs and necessary disbursements in addition to any
other relief to which such party may be entitled.

      4.7 Amendments and Waivers. Any term of this Agreement may be amended and
the observance of any term of this Agreement may be waived (either generally or
in a particular instance and either retroactively or prospectively), only with
the written consent of Acusphere and the holders of at least 55% of the
Registrable Securities then outstanding (provided that if an amendment or waiver
affects any Investor in a manner that is different from the effect of such
waiver or amendment on all other Investors, then the consent of such Investor
shall be required for such amendment or waiver, and further provided that no
such amendment or waiver will be effective to increase any obligations of Oberg
without her written consent). Any amendment or waiver effected in accordance
with this paragraph shall be binding upon each holder of any Registrable
Securities then outstanding, each future holder of all such Registrable
Securities, and Acusphere.

      4.8 Severability. If one or more provisions of this Agreement are held to
be unenforceable under applicable law, such provision shall be excluded from
this Agreement and the balance of the Agreement shall be interpreted as if such
provision were so excluded and shall be enforceable in accordance with its
terms.

      4.9 Aggregation of Stock. All shares of Registrable Securities held or
acquired by affiliated entities or persons shall be aggregated together for the
purpose of determining the availability of any rights under this Agreement.

      4.10 Entire Agreement. This Agreement (including the Exhibits and
Schedules hereto, if any) constitutes the full and entire understanding and
agreement between the parties with regard to the subjects hereof and thereof.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]
<PAGE>
                               AMENDMENT NO. 1 TO
                           TENTH AMENDED AND RESTATED
             INVESTORS' RIGHTS AGREEMENT, DATED AS OF APRIL 11, 2003

      (a)   Section 3.7 is hereby deleted in its entirety and replaced with the
            following:

            "3.7. Termination of Certain Covenants. The provisions set forth in
            Sections 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.8, 3.9, 3.10 and 3.11 shall
            terminate and be of no further force or effect upon the earlier of
            (i) the closing of a firm commitment underwritten public offering
            pursuant to an effective registration statement under the Securities
            Act covering the offer and sale of Acusphere's Common Stock and (ii)
            the closing of a transaction or series of transactions resulting in
            a Change of Control, as defined in Acusphere's Amended and Restated
            Certificate of Incorporation, as amended and in effect from time to
            time."

      (b)   Except as hereby amended, the Investors' Rights Agreement shall
            remain in full force and effect.
<PAGE>
      IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.


                                  ACUSPHERE, INC.
                                  500 Arsenal Street
                                  Watertown, MA 02472


                                  By: /s/ Sherri C. Oberg
                                      ------------------------------------------
                                      Sherri C. Oberg
                                      President and Chief Executive Officer
<PAGE>
                                  MVI MEDICAL VENTURE
                                  INVESTMENTS LIMITED
                                  c/o New Medical Technologies
                                  Attention: Beat R. Merz, Dr. sc. Techn.
                                  Elisabethenstrasse 23
                                  CH-4051 Basel
                                  Switzerland


                                  By: /s/ John Arnold
                                      ------------------------------------------
                                      Name: John Arnold
                                      Title: Director
<PAGE>
                                  DIAMOND CAPITAL MANAGEMENT, INC. (as
                                  Investment Advisor for the Retirement
                                  Program Plan for Employees of Union
                                  Carbide Corporation and its
                                  Participating Subsidiary Companies)

                                  c/o The Dow Chemical Company
                                  1320 Waldo Avenue, DORINCO 100
                                  Midland, MI 48674



                                  By: /s/ G. N. van Drunen
                                      ------------------------------------------
                                      Name: G. N. van Drunen
                                      Title: Director of Alternative Investments
<PAGE>
                                  BANK OF AMERICA VENTURES
                                  950 Tower Lane, Suite 700
                                  Foster City, CA 94404

                                  By: /s/ Kate D. Mitchell
                                      ----------------------------------------
                                      Name: Kate D. Mitchell
                                      Title: Managing Director


                                  BA VENTURE PARTNERS II
                                  c/o BankAmerica Ventures
                                  950 Tower Lane, Suite 700
                                  Foster City, CA 94404

                                  By: /s/ Robert M. Obuch
                                      ----------------------------------------
                                      Name: Robert M. Obuch
                                      Title: General Partner
<PAGE>
                                  ALTA V LIMITED PARTNERSHIP
                                  200 Clarendon Street, 51st Floor
                                  Boston, MA 02116

                                  By: Alta V Management Partners, L.P.

                                  By: /s/ Eileen McCarthy
                                      ------------------------------------------
                                      Name: Eileen McCarthy
                                      Title: General Partner

                                  CUSTOMS HOUSE PARTNERS
                                  200 Clarendon Street, 51st Floor
                                  Boston, MA 02116


                                  By: /s/ Eileen McCarthy
                                      ------------------------------------------
                                      Name: Eileen McCarthy
                                      Title: Under Power of Attorney
<PAGE>
                                  POLARIS VENTURE PARTNERS, L.P.
                                  1000 Winter Street
                                  Suite 3350
                                  Waltham, MA 02154

                                  By: Polaris Venture Management Co., LLC,
                                      Its General Partner


                                  By: /s/ William E. Bilodeau
                                      ------------------------------------------
                                      Name: William E. Bilodeau
                                      Title: Attorney-in-fact

                                  POLARIS VENTURE PARTNERS FOUNDERS' FUND, L.P.
                                  1000 Winter Street
                                  Suite 3350
                                  Waltham, MA 02154

                                  By: Polaris Venture Management Co., LLC
                                      Its General Partner


                                  By: /s/ William E. Bilodeau
                                      ------------------------------------------
                                      Name: William E. Bilodeau
                                      Title: Attorney-in-fact
<PAGE>
                                  BANCBOSTON VENTURES, INC.
                                  175 Federal Street, 10th Floor
                                  Boston, MA 02110


                                  By: /s/ John B. McCormick
                                      ------------------------------------------
                                      Name: John B. McCormick
                                      Title: Vice President
<PAGE>
                                  PRIVATE EQUITY PORTFOLIO FUND II, LLC
                                  c/o BancBoston Capital
                                  175 Federal Street, 10th Floor
                                  Boston, MA 02110

                                  By: Fleet National Bank NA,
                                      Its Manager

                                  By: /s/ John DeSisto
                                      ------------------------------------------
                                      Name: John DeSisto
                                      Title: Vice President
<PAGE>
                                  TECHNOLOGY FUNDING PARTNERS III, L.P.,
                                  A Delaware Limited Partnership
                                  c/o Debra Giambruno
                                  1107 Investment Blvd., Suite 180
                                  El Dorado Hills, CA 95762

                                  By: Technology Funding, Inc.
                                      Managing General Partner

                                  By: /s/ Peter F. Bernardoni
                                      ------------------------------------------
                                      Vice President


                                  TECHNOLOGY FUNDING VENTURE
                                  PARTNERS V, AN AGGRESSIVE GROWTH FUND L.P.
                                  c/o Debra Giambruno
                                  1107 Investment Blvd., Suite 180
                                  El Dorado Hills, CA 95762


                                  By: Technology Funding, Inc.
                                      Managing General Partner

                                  By: /s/ Peter F. Bernardoni
                                      ------------------------------------------
                                      Vice President
<PAGE>
                                  DEBAR INVESTMENT PARTNERSHIP LLP
                                  c/o Sue DeCarlo
                                  5 Misty Brook Lane
                                  New Fairfield, CT 06812


                                  By: /s/ Sue DeCarlo
                                      ------------------------------------------
                                      Name: Sue DeCarlo
                                      Title: Managing Member



<PAGE>

                                  AUDAX PRIVATE EQUITY FUND, L.P.
                                  c/o Audax Group
                                  101 Huntington Ave.
                                  Floor 24
                                  Boston, MA 02199

                                  By:     Audax Private Equity Business, LLC
                                          Its General Partner

                                  By:     Audax Private Equity Business, L.P.,
                                          Its Managing Member

                                  By:     /s/ Steven F. Kaplan
                                          ------------------------
                                           Name: Steven F. Kaplan
                                           Title:


                                  AUDAX CO-INVEST, L.P.
                                  c/o Audax Group
                                  101 Huntington Ave.
                                  Floor 24
                                  Boston, MA 02199

                                  By:     101 Huntington Holdings, LLC
                                          Title: General Partner

                                  By:     /s/ Steven F. Kaplan
                                          ---------------------------
                                          Name: Steven F. Kaplan
                                          Title: Authorized Signatory


<PAGE>



                                  AUDAX TRUST CO-INVEST, L.P.
                                  c/o Audax Group
                                  101 Huntington Ave.
                                  Floor 24
                                  Boston, MA 02199

                                  By:     101 Huntington Holdings, LLC
                                          Title: General Partner

                                  By:     /s/ Steven F. Kaplan
                                          ---------------------------
                                          Name: Steven F. Kaplan
                                          Title: Authorized Signatory


                                  AFF CO-INVEST, L.P.
                                  c/o Audax Group
                                  101 Huntington Ave.
                                  Floor 24
                                  Boston, MA 02199

                                  By:     101 Huntington Holdings, LLC
                                          Title: General Partner

                                  By:     /s/ Steven F. Kaplan
                                          ---------------------------
                                          Name: Steven F. Kaplan
                                          Title: Authorized Signatory


<PAGE>



                                  AUDAX SPECIAL PURPOSE CO-INVEST, L.P.
                                  c/o Audax Group
                                  101 Huntington Ave.
                                  Floor 24
                                  Boston, MA 02199

                                  By:     101 Huntington Holdings, LLC
                                          Title: General Partner

                                  By:     /s/ Steven F. Kaplan
                                          ---------------------------
                                          Name: Steven F. Kaplan
                                          Title: Authorized Signatory





<PAGE>



                                  THOMAS WEISEL CAPITAL PARTNERS, L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partner

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member

                                  By:  /s/ Derek Lemke-von Ammon
                                       ------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact


                                  TWP CEO FOUNDERS CIRCLE (AI), L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partner

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member


                                  By:  /s/ Derek Lemke-von Ammon
                                       -----------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact



<PAGE>


                                  TWP CEO FOUNDERS CIRCLE (QP), L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partner

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member

                                  By:  /s/ Derek Lemke-von Ammon
                                       -------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact


                                  THOMAS WEISEL CAPITAL PARTNERS (DUTCH), L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partner

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member

                                  By:  Thomas Weisel Capital Partners Group
                                       LLC, its managing member

                                  By:  /s/ Derek Lemke-von Ammon
                                       ------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact


<PAGE>


                                  THOMAS WEISEL CAPITAL PARTNERS
                                  (DUTCH II), L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners
                                       (Dutch), LLC, its general partner

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its managing member

                                  By:  Thomas Weisel Capital Partners Group
                                       LLC, its managing member

                                  By:  /s/ Derek Lemke-von Ammon
                                       ------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact

                                  THOMAS WEISEL CAPITAL PARTNERS EMPLOYEE
                                  FUND, L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partner

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member


                                  By:  /s/ Derek Lemke-von Ammon
                                       -------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact




<PAGE>


                                  TWP 2000 CO-INVESTMENT FUND, L.P.
                                  c/o Derek Lemke-von Ammon, Executive
                                  Committee, Director of Private Equity
                                  c/o One Montgomery Street, Suite 3700
                                  San Francisco, California 94104

                                  By:  Thomas Weisel Capital Partners LLC,
                                       its general partners

                                  By:  Thomas Weisel Partners Group LLC,
                                       its managing member


                                  By:  /s/ Derek Lemke-von Ammon
                                       ------------------------------------
                                       Derek Lemke-von Ammon
                                       Attorney-in-fact



<PAGE>


                                  DINTERSMITH FAMILY LIMITED PARTNERSHIP
                                  c/o Charles River Ventures
                                  Bay Colony Corporate Center
                                  1000 Winter Street
                                  Suite 3300
                                  Waltham, MA 02154


                                  By:  /s/ Ted R. Dintersmith
                                       ---------------------------------------
                                       Name:  Ted Dintersmith
                                       Title:    Managing Partner



<PAGE>


                                  ROBERT S. LANGER
                                  98 Montvale Road
                                  Newton, MA 02459

                                   /s/ Robert S. Langer
                                  --------------------------




<PAGE>


                                  KD VENTURES LLC
                                  c/o 26 Turtle Rock Court
                                  Tiburon, CA 94920


                                  By:  /s/ David Norman
                                       ---------------------------------------
                                       Name: David Norman
                                       Title:   Director




<PAGE>



                                  FOR THE PURPOSES OF SECTIONS 1, 3.4 AND 4
                                  ONLY

                                  SHERRI C. OBERG
                                  157 Bristol Road
                                  Wellesley, MA 02181

                                  /s/ Sherri C. Oberg
                                  --------------------------------




<PAGE>


                                                                      Schedule A

                                  Investors

      Thomas Weisel Capital Partners, L.P.
      TWP CEO Founders Circle (AI), L.P.
      TWP CEO Founders Circle (QP), L.P.
      Thomas Weisel Capital Partners (Dutch), L.P.
      Thomas Weisel Capital Partners (Dutch II), L.P.
      Thomas Weisel Capital Partners Employee Fund, L.P.
      TWP 2000 Co-Investment Fund, L.P.
      MVI Medical Venture Investments Limited
      Diamond Capital Management, Inc.
         (as Investment Advisor for the Retirement Program Plan for Employees
         of Union Carbide Corporation and its Participating Subsidiary
         Companies)
      Audax Private Equity Fund, L.P.
      Audax Co-Invest, L.P.
      Audax Trust Co-Invest, L.P.
      AFF Co-Invest, L.P.
      Audax Special Purpose Co-Invest, L.P.
      DeBar Investment Partnership LLP
      Prism Venture Partners I, L.P.
      The CIT Group/Equity Investments, Inc.
      ABS Employees' Venture Fund L.P.
      Bank of America Ventures
      BA Venture Partners II
      Alta V Limited Partnership
      Customs House Partners
      Polaris Venture Partners, L.P.
      Polaris Venture Partners Founders' Fund, L.P.
      The Venture Capital Fund of New England III, L.P.
      Trustees of Boston University
      BancBoston Ventures, Inc.
      Private Equity Portfolio Fund II, LLC
      Technology Funding Venture Partners V, An Aggressive Growth Fund, L.P.
      Technology Medical Partners I, L.P.
      Technology Funding Partners III, L.P.
      Dintersmith Family Limited Partnership
      Robert S. Langer
      KD Ventures LLC
      A.R. Levinson Trust No. 1, dated July 15, 1990, as amended
      Alexandria Real Estate Equities, L.P.
      GATX Ventures, Inc.
      Venture Lending & Leasing III, Inc.