<PAGE>
                                                                   Exhibit 10.21

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 AS
AMENDED, OR ANY STATE SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE,
PLEDGED, OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
RELATED THERETO OR AN OPINION OF COUNSEL (WHICH MAY BE COMPANY COUNSEL)
REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED
UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR ANY APPLICABLE STATE SECURITIES
LAWS.


                                WARRANT AGREEMENT

  TO PURCHASE SHARES OF THE SERIES F NON-VOTING CONVERTIBLE PREFERRED STOCK OF

                                 ACUSPHERE, INC.

                DATED AS OF MARCH 30, 2001 (THE "EFFECTIVE DATE")


      WHEREAS, Acusphere, Inc., a Delaware corporation (the "Company"), has
entered into a Lease Agreement dated as of the date hereof with ARE-500
Arsenal Street, LLC, an affiliate of Alexandria Real Estate Equities, L.P., a
Delaware limited partnership (the "Warrantholder"); and

      WHEREAS, the Company desires to grant to Warrantholder, in consideration
of entering into such Lease Agreement, the right to purchase shares of its
Series F Non-Voting Convertible Preferred Stock;

      NOW, THEREFORE, in consideration of the Warrantholder executing and
delivering such Lease Agreement and in consideration of mutual covenants and
agreements contained herein, the Company and Warrantholder agree as follows:

1.    GRANT OF THE RIGHT TO PURCHASE PREFERRED STOCK.


      The Company hereby grants to the Warrantholder, and the Warrantholder is
entitled, upon the terms and subject to the conditions hereinafter set forth, to
subscribe for and purchase from the Company 150,000 fully paid and
non-assessable shares of the Company's Series F Non-Voting Convertible Preferred
Stock (the "PREFERRED STOCK") at a purchase price of $4.75 per share (the
"EXERCISE PRICE"). The number and purchase price of such shares are subject to
adjustment as provided in Section 8 hereof.

2.    TERM OF THE WARRANT AGREEMENT.

      Except as otherwise provided for herein, the term of this Warrant
Agreement and the right to purchase Preferred Stock as granted herein shall
commence on the Effective Date and shall be exercisable for a period of ten (10)
years.

3.    EXERCISE OF THE PURCHASE RIGHTS.

      (a) EXERCISE. The purchase rights set forth in this Warrant Agreement are
exercisable by the Warrantholder, in whole or in part, at any time, or from time
to time prior to the expiration of the term set forth in Section 2 above, by
tendering to the Company at its principal office a notice of exercise in the
form attached hereto as EXHIBIT I (the "Notice of Exercise"), duly completed and
executed. Promptly upon receipt of the Notice of Exercise and the payment of the
purchase price in accordance with the terms set forth below, and in no event
later than twenty-one (21) days thereafter, the Company shall issue to the
Warrantholder a certificate for the number of shares of Preferred Stock
purchased and shall execute the Notice of Exercise indicating the number of
shares which remain subject to future purchases, if any.
<PAGE>
                                      -2-

      The Exercise Price may be paid at the Warrantholder's election either (i)
by cash or check, or (ii) by surrender of Warrants ("Net Issuance") as
determined below. If the Warrantholder elects the Net Issuance method, the
Company shall issue Preferred Stock in accordance with the following formula:

                  X = Y(A-B)
                      ------
                        A

      WHERE:      X = the number of shares of Preferred Stock to be issued
                      to the Warrantholder.

                  Y = the number of shares of Preferred Stock requested to
                      be exercised under this Warrant Agreement.

                  A = the fair market value of one (1) share of Preferred Stock.

                  B = the Exercise Price.

      For purposes of the above calculation, current fair market value of
Preferred Stock shall mean with respect to each share of Preferred Stock:

            (i) if the exercise is in connection with an initial public offering
      of the Company's Common Stock, and if the Company's Registration Statement
      relating to such public offering has been declared effective by the SEC,
      then the fair market value per share shall be the initial "Price to
      Public" specified in the final prospectus with respect to the offering;

            (ii) if this Warrant is exercised after, and not in connection with
      the Company's initial public offering, and:

                        (a) if traded on a securities exchange or the Nasdaq
            National Market, the fair market value per share shall be deemed to
            be the average of the closing prices over a twenty-one (21) day
            period ending three days before the day the current fair market
            value of the securities is being determined; or

                        (b) if actively traded over-the-counter, the fair market
            value shall be deemed to be the average of the closing bid and asked
            prices quoted on the NASDAQ system (or similar system) over the
            twenty-one (21) day period ending three days before the day the
            current fair market value of the securities is being determined;

            (iii) if at any time the Preferred Stock is not listed on any
      securities exchange or quoted in the NASDAQ System or the over-the-counter
      market, the current fair market value of Preferred Stock shall be the
      highest price per share which the Company could obtain from a willing
      buyer (not a current employee or director) for shares of Preferred Stock
      sold by the Company, from authorized but unissued shares, as determined in
      good faith by its Board of Directors, unless the Company shall become
      subject to a merger, acquisition or other consolidation pursuant to which
      the Company is not the surviving party, in which case the fair market
      value of Preferred Stock shall be deemed to be the value received by the
      holders of the Company's Preferred Stock on a common equivalent basis
      pursuant to such merger or acquisition.

      Upon partial exercise by either cash or Net Issuance, the Company shall
promptly issue an amended Warrant Agreement representing the remaining number of
shares purchasable hereunder. All other terms and conditions of such amended
Warrant Agreement shall be identical to those contained herein, including, but
not limited to the Effective Date hereof.

4.    RESERVATION OF SHARES.

<PAGE>
                                      -3-
      During the term of this Warrant Agreement, the Company shall at all times
have authorized and reserved a sufficient number of shares of its Preferred
Stock to provide for the exercise of the rights to purchase Preferred Stock as
provided for herein.

5.    NO FRACTIONAL SHARES OR SCRIP.


      No fractional shares or scrip representing fractional shares shall be
issued upon the exercise of the Warrant, but in lieu of such fractional shares
the Company shall make a cash payment therefor upon the basis of the Exercise
Price then in effect.

6.    NO RIGHTS AS SHAREHOLDER.


      This Warrant Agreement does not entitle the Warrantholder to any voting
rights or other rights as a shareholder of the Company prior to the exercise of
the Warrant.

7.    WARRANTHOLDER REGISTRY.


      The Company shall maintain a registry showing the name and address of the
registered holder of this Warrant Agreement.

8.    ADJUSTMENT RIGHTS.


      The purchase price per share and the number of shares of Preferred Stock
purchasable hereunder are subject to adjustment, as follows:

      (a) MERGER AND SALE OF ASSETS. If at any time there shall be a capital
reorganization of the shares of the Company's stock (other than a combination,
reclassification, exchange or subdivision of shares otherwise provided for
herein), or a merger or consolidation of the Company with or into another
corporation whether or not the Company is the surviving corporation, or the sale
of all or substantially all of the Company's properties and assets to any other
person (hereinafter referred to as a "MERGER EVENT"), then, as a part of such
Merger Event, lawful provision shall be made so that the Warrantholder shall
thereafter be entitled to receive, upon exercise of the Warrant, the number of
shares of preferred stock, other securities, cash or property of the successor
corporation resulting from such Merger Event, equivalent in value to that which
would have been issuable if Warrantholder had exercised this Warrant immediately
prior to the Merger Event. In any such case, appropriate adjustment (as
determined in good faith by the Company's Board of Directors) shall be made in
the application of the provisions of this Warrant Agreement with respect to the
rights and interest of the Warrantholder after the Merger Event to the end that
the provisions of this Warrant Agreement (including adjustments of the Exercise
Price and number of shares of Preferred Stock purchasable) shall be applicable
to the greatest extent possible.

      (b) RECLASSIFICATION OF SHARES. If the Company at any time shall, by
combination, reclassification, exchange or subdivision of securities or
otherwise, change any of the securities as to which purchase rights under this
Warrant Agreement exist into the same or a different number of securities of any
other class or classes, this Warrant Agreement shall thereafter represent the
right to acquire such number and kind of securities as would have been issuable
as the result of such change with respect to the securities which were subject
to the purchase rights under this Warrant Agreement immediately prior to such
combination, reclassification, exchange, subdivision or other change. If at any
time all of the outstanding shares of Preferred Stock have been converted to
shares of the Company's Common Stock, then, from and after the date of such
conversion, (i) this Warrant Agreement shall no longer be exercisable for
Preferred Stock, but shall represent the right to acquire that number of shares
of Common Stock as shall be equal to the number of shares issuable upon
conversion of the Preferred Stock which would have been issued on exercise of
this Warrant Agreement had such exercise occurred immediately prior to such
conversion, (ii) the Exercise Price shall be immediately adjusted to equal the
quotient obtained by dividing (A) the aggregate Exercise Price of the maximum
number of shares of Preferred Stock for which this Warrant Agreement was
exercisable immediately prior to such conversion, by (B) the maximum number of
shares of Common Stock for
<PAGE>
                                      -4-

which this Warrant Agreement is exercisable immediately after such conversion,
and (iii) the term "Preferred Stock," as used in this Warrant Agreement, shall
be deemed appropriately amended to refer to "Common Stock."

      (c) SUBDIVISION OR COMBINATION OF SHARES. If the Company at any time shall
combine or subdivide its Preferred Stock, the Exercise Price shall be
proportionately decreased in the case of a subdivision, or proportionately
increased in the case of a combination.

      (d) STOCK DIVIDENDS. If the Company at any time shall pay a dividend
payable in, or make any other distribution (except any distribution specifically
provided for in the foregoing subsections (a) or (b)) of the Company's Preferred
Stock, then the Exercise Price shall be adjusted, from and after the record date
of such dividend or distribution, to that price determined by multiplying the
Exercise Price in effect immediately prior to such record date by a fraction (i)
the numerator of which shall be the total number of all shares of the Company's
stock outstanding immediately prior to such dividend or distribution, and (ii)
the denominator of which shall be the total number of all shares of the
Company's stock outstanding immediately after such dividend or distribution. The
Warrantholder shall thereafter be entitled to purchase, at the Exercise Price
resulting from such adjustment, the number of shares of Preferred Stock
(calculated to the nearest whole share) obtained by multiplying the Exercise
Price in effect immediately prior to such adjustment by the number of shares of
Preferred Stock issuable upon the exercise hereof immediately prior to such
adjustment and dividing the product thereof by the Exercise Price resulting from
such adjustment.

      (e) LIMITATION ON ADJUSTMENTS. To the extent that any adjustment under
Sections 8(a), (b), (c) or (d) hereunder would be duplicative of a comparable
adjustment made by operation of the Charter (defined below) or applicable law,
no such duplicative adjustment shall be made hereunder.

      (f) ANTIDILUTION RIGHTS. Additional antidilution rights applicable to the
Preferred Stock purchasable hereunder are as set forth in the Company's
Certificate of Incorporation, as amended through the Effective Date, a true and
complete copy of which is attached hereto as EXHIBIT IV (the "Charter"). The
Company shall promptly provide the Warrantholder with any restatement,
amendment, modification or waiver of the Charter that results in an adjustment
hereunder. The Company shall provide Warrantholder with prior written notice of
any issuance of its stock or other equity security to occur after the Effective
Date of this Warrant that results in an adjustment hereunder, which notice shall
include (a) the price at which such stock or security is to be sold, (b) the
number of shares to be issued, and (c) such other information as necessary for
Warrantholder to determine if a dilutive event has occurred.

      (g) NOTICE. If: (i) the Company shall declare any dividend or distribution
upon its stock, whether in cash, property, stock or other securities, excluding
any dividend covered by one of the preceding adjustments; (ii) the Company shall
offer for subscription prorata to the holders of any class of its Preferred or
other convertible stock (in their capacity as such and not pursuant to
contractual purchase rights) any additional shares of stock of any class or
other rights; (iii) there shall be any Merger Event; or (iv) there shall be any
voluntary dissolution, liquidation or winding up of the Company; or (v) there
shall occur any redemption of the Preferred Stock pursuant to the provisions of
Section 3 of the Company's Certificate of Incorporation then, in connection with
each such event, the Company shall send to the Warrantholder: (A) at least ten
(10) days' prior written notice of the date on which the books of the Company
shall close or a record shall be taken for such dividend, distribution,
subscription rights or redemption (specifying the date on which the holders of
Preferred Stock shall be entitled thereto) or for determining rights to vote in
respect of such Merger Event, dissolution, liquidation or winding up; and (B) in
the case of any such Merger Event, dissolution, liquidation or winding up, at
least ten (10) days' prior written notice of the date when the same shall take
place (and specifying the date on which the holders of Preferred Stock shall be
entitled to exchange their Preferred Stock for securities or other property
deliverable upon such Merger Event, dissolution, liquidation or winding up). In
the case of an initial public offering, the Company shall give Warrantholder
notice of filing of a registration statement with the SEC promptly after such
filing, but in no event later than three (3) business days after such filing.
Each such written notice shall set forth, in reasonable detail, (i) the event
requiring the adjustment, (ii) the amount of the adjustment, if any, (iii) the
method by which such adjustment was calculated, (iv) the Exercise Price, and (v)
the number of shares subject to purchase hereunder after
<PAGE>
                                      -5-

giving effect to such adjustment, and shall be given by first class mail,
postage prepaid, addressed to the Warrantholder, at the address as shown on the
books of the Company.

      (h) TIMELY NOTICE. Failure to timely provide such notice required by
subsection (g) above shall entitle Warrantholder to retain the benefit of the
applicable notice period notwithstanding anything to the contrary contained in
any insufficient notice received by Warrantholder. The notice period shall begin
on the date Warrantholder actually receives a written notice containing all the
information specified above.

9.    REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY.


      9.1 Authorization. The Company has full power and authority to enter into
this Warrant Agreement. This Warrant Agreement has been duly authorized,
executed and delivered by the Company and constitutes its valid and legally
binding obligation, enforceable in accordance with its terms.

      9.2 Reservation of Preferred Stock. The Company shall at all times reserve
and keep available out of its authorized but unissued shares of Series F
Non-Voting Convertible Preferred Stock, solely for the purpose of effecting the
exercise of this Warrant Agreement, such number of its shares of Series F
Non-Voting Convertible Preferred Stock, free from preemptive rights, as shall
from time to time be sufficient to effect the exercise of this Warrant
Agreement, and if at any time the number of authorized but unissued shares of
Series F Non-Voting Convertible Preferred Stock shall not be sufficient to
effect the exercise of the entire Warrant Agreement, in addition to such other
remedies as shall be available to the holder of this Warrant Agreement, the
Company shall take such action as may be necessary to increase its authorized
but unissued shares of Series F Non-Voting Convertible Preferred Stock to such
number of shares as shall be sufficient for such purposes.

      9.3 Adjustment in Number of Shares Issuable and Purchase Price. There has
not been nor will there be any adjustment to the number of shares issuable or
the purchase price payable upon the exercise of any securities of the Company
convertible into or exchangeable for shares of Series F Non-Voting Convertible
Preferred Stock resulting from the issuance or exercise of this Warrant
Agreement.

      9.4 Valid Issuance. This Warrant Agreement, when issued and delivered in
accordance with the terms hereof will be duly authorized and validly issued, and
the Series F Non-Voting Convertible Preferred Stock issuable upon the exercise
hereof, when issued pursuant to the terms hereof and upon payment of the
exercise price, shall, upon issuance, be duly authorized, validly issued, full
paid and nonassessable.

10.   REPRESENTATIONS AND COVENANTS OF THE WARRANTHOLDER.


      This Warrant Agreement has been entered into by the Company in reliance
upon the following representations and covenants of the Warrantholder:

      (a) INVESTMENT PURPOSE. The right to acquire Preferred Stock or the
Preferred Stock issuable upon exercise of the Warrantholder's rights contained
herein will be acquired for investment and not with a view to the sale or
distribution of any part thereof, and the Warrantholder has no present intention
of selling or engaging in any public distribution of the same except pursuant to
a registration or exemption.

      (b) PRIVATE ISSUE. The Warrantholder understands (i) that the Preferred
Stock issuable upon exercise of this Warrant is not registered under the 1933
Act or qualified under applicable state securities laws on the ground that the
issuance contemplated by this Warrant Agreement will be exempt from the
registration and qualifications requirements thereof, and (ii) that the
Company's reliance on such exemption is predicated on the representations set
forth in this Section 10.

      (c) DISPOSITION OF WARRANTHOLDER'S RIGHTS. In no event shall the
Warrantholder make a disposition of any of its rights to acquire Preferred Stock
or Preferred Stock issuable upon exercise of such rights unless and until (i) it
shall have notified the Company of the proposed disposition, and (ii) if
requested by the Company, it shall have furnished the Company with an opinion of
counsel (which counsel may either be inside or outside counsel to
<PAGE>
                                      -6-

the Warrantholder) satisfactory to the Company and its counsel to the effect
that (A) appropriate action necessary for compliance with the 1933 Act has been
taken, or (B) an exemption from the registration requirements of the 1933 Act is
available. Notwithstanding the foregoing, the restrictions imposed upon the
transferability of any of its rights to acquire Preferred Stock or Preferred
Stock issuable on the exercise of such rights do not apply to transfers from the
beneficial owner of any of the aforementioned securities to its nominee or from
such nominee to its beneficial owner, and shall terminate as to any particular
share of Preferred Stock when (1) such security shall have been effectively
registered under the 1933 Act and sold by the holder thereof in accordance with
such registration or (2) such security shall have been sold without registration
in compliance with Rule 144 under the 1933 Act, or (3) a letter shall have been
issued to the Warrantholder at its request by the staff of the Securities and
Exchange Commission or a ruling shall have been issued to the Warrantholder at
its request by such Commission stating that no action shall be recommended by
such staff or taken by such Commission, as the case may be, if such security is
transferred without registration under the 1933 Act in accordance with the
conditions set forth in such letter or ruling and such letter or ruling
specifies that no subsequent restrictions on transfer are required. Whenever the
restrictions imposed hereunder shall terminate, as hereinabove provided, the
Warrantholder or holder of a share of Preferred Stock then outstanding as to
which such restrictions have terminated shall be entitled to receive from the
Company, without expense to such holder, one or more new certificates for the
Warrant or for such shares of Preferred Stock not bearing any restrictive
legend.

      (d)   FINANCIAL RISK.  The Warrantholder has such knowledge and
experience in financial and business matters as to be capable of evaluating
the merits and risks of its investment, and has the ability to bear the
economic risks of its investment.

      (e) RISK OF NO REGISTRATION. The Warrantholder understands that if the
Company does not register with the Securities and Exchange Commission pursuant
to Section 12 of the 1934 Act (the "1934 Act"), or file reports pursuant to
Section 15(d), of the 1934 Act, or if a registration statement covering the
securities under the 1933 Act is not in effect when it desires to sell (i) the
rights to purchase Preferred Stock pursuant to this Warrant Agreement, or (ii)
the Preferred Stock issuable upon exercise of the right to purchase, it may be
required to hold such securities for an indefinite period. The Warrantholder
also understands that any sale of its rights of the Warrantholder to purchase
Preferred Stock or Preferred Stock which might be made by it in reliance upon
Rule 144 under the 1933 Act may be made only in accordance with the terms and
conditions of that Rule.

      (f)   ACCREDITED INVESTOR.  Warrantholder is an "accredited investor"
within the meaning of the Securities and Exchange Rule 501 of Regulation D,
as presently in effect.

11.    TRANSFERS.

      Subject to the terms and conditions contained in Section 10 hereof, this
Warrant Agreement and all rights hereunder are transferable in whole or in part
by the Warrantholder and any successor transferee, provided, however, in no
event shall the number of transfers of the rights and interests in all of the
Warrants exceed three (3) transfers (other than transfers to affiliates of, or
entities which control, are controlled by, or under common control with,
Alexandria Real Estate Equities, Inc.), or be to a direct competitor of the
Company or be to a person who is not an "accredited investor" under the
Securities Act and the rules and regulations thereunder. The same restrictions
shall apply to shares issued upon exercise of the Warrants until the Company has
completed its initial public offering. The transfer shall be recorded on the
books of the Company upon receipt by the Company of a notice of transfer in the
form attached hereto as EXHIBIT III (the "Transfer Notice"), at its principal
offices and the payment to the Company of all transfer taxes and other
governmental charges imposed on such transfer.

12.   MISCELLANEOUS.

      (a) EFFECTIVE DATE. The provisions of this Warrant Agreement shall be
construed and shall be given effect in all respects as if it had been executed
and delivered by the Company on the date hereof. This Warrant Agreement shall be
binding upon any successors or assigns of the Company.
<PAGE>
                                      -7-

      (b) ATTORNEY'S FEES. In any litigation, arbitration or court proceeding
between the Company and the Warrantholder relating hereto, the prevailing party
shall be entitled to attorneys' fees and expenses and all costs of proceedings
incurred in enforcing this Warrant Agreement.

      (c)   GOVERNING LAW.  This Warrant Agreement shall be governed by and
construed for all purposes under and in accordance with the laws of the State
of Delaware.

      (d) COUNTERPARTS. This Warrant 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.

      (e) NOTICES. Any notice required or permitted hereunder shall be given in
writing and shall be deemed effectively given upon personal delivery, facsimile
transmission (provided that the original is sent by personal delivery or mail as
hereinafter set forth) or seven (7) days after deposit in the United States
mail, by registered or certified mail, addressed (i) TO THE WARRANTHOLDER at its
address as shown on the books of the Company, with a copy to Mayer, Brown &
Platt, 350 South Grand Avenue, 25th Floor, Los Angeles, California 90071,
Attention: James R. Walther, Esq. and (ii) TO THE COMPANY at 38 Sidney Street,
Cambridge, MA 02139, Attention: Steven Delaney, Vice President Finance &
Administration (and/or if by Facsimile, (617) 577-0233) or at such other address
as any such party may subsequently designate by written notice to the other
party.

      (f) REMEDIES. In the event of any default hereunder, the non-defaulting
party may proceed to protect and enforce its rights either by suit in equity
and/or by action at law, including but not limited to an action for damages as a
result of any such default, and/or an action for specific performance for any
default where Warrantholder will not have an adequate remedy at law and where
damages will not be readily ascertainable. The Company expressly agrees that it
shall not oppose an application by the Warrantholder or any other person
entitled to the benefit of this Agreement requiring specific performance of any
or all provisions hereof or enjoining the Company from continuing to commit any
such breach of this Agreement.

      (g) NO IMPAIRMENT OF RIGHTS. The Company will not, by amendment of its
Charter or through any other means, avoid or seek to avoid the observance or
performance of any of the terms of this Warrant, but will at all times in good
faith assist in the carrying out of all such terms and in the taking of all such
actions as may be necessary or appropriate in order to protect the rights of the
Warrantholder against impairment.

      (h) SURVIVAL. The representations, warranties, covenants and conditions of
the respective parties contained herein or made pursuant to this Warrant
Agreement shall survive the execution and delivery of this Warrant Agreement.

      (i) SEVERABILITY. In the event any one or more of the provisions of this
Warrant Agreement shall for any reason be held invalid, illegal or
unenforceable, the remaining provisions of this Warrant Agreement shall be
unimpaired, and the invalid, illegal or unenforceable provision shall be
replaced by a mutually acceptable valid, legal and enforceable provision, which
comes closest to the intention of the parties underlying the invalid, illegal or
unenforceable provision.

      (j)   AMENDMENTS.  Any provision of this Warrant Agreement may be
amended by a written instrument signed by the Company and by the
Warrantholder.


                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

<PAGE>
                                      -8-


      IN WITNESS WHEREOF, the parties hereto have caused this Warrant Agreement
to be executed by its officers thereunto duly authorized as of the Effective
Date.

      COMPANY:                ACUSPHERE, INC.


                              By:/s/ Sherri Oberg
                                 ____________________________

                              Title: President and CEO

      WARRANTHOLDER:          ALEXANDRIA REAL ESTATE EQUITIES, L.P.


                              By:   ARE-QRS CORP.,
                                    its general partner



                              By:/s/ Laurie A. Allen
                                 _____________________________

                              Title: Senior Vice President, Business
                                     Development and Legal Affairs
<PAGE>
                                      -9-

                                    EXHIBIT I

                               NOTICE OF EXERCISE


TO:   ____________________________

(1)   The undersigned Warrantholder hereby elects to purchase _______ shares of
      the ____ Stock of _________________, pursuant to the terms of the Warrant
      Agreement dated the ______ day of ________________________, ________ (the
      "Warrant Agreement") between _____________________________________ and the
      Warrantholder, and tenders herewith payment of the purchase price (by cash
      or thorough exercise of the "cashless exercise" provisions of Section 3(a)
      of the Warrant) for such shares in full, together with all applicable
      transfer taxes, if any.

(2)   In exercising its rights to purchase the _____________________ Stock of
      _____________________, the undersigned hereby confirms and acknowledges
      the investment representations and warranties made in Section 10 of the
      Warrant Agreement.

(3)   Please issue a certificate or certificates representing said shares of
      ____________Stock in the name of the undersigned or in such other name as
      is specified below.




                              ____________________________________
                              (Name)

                              ____________________________________
                             (Address)


      WARRANTHOLDER:                ALEXANDRIA REAL ESTATE EQUITIES, L.P.

                                    ARE-QRS CORP.,
                                    its general partner

                                    By:____________________________

                                    Title:_________________________

                                    Date: _________________________


<PAGE>
                                      -10-

                                   EXHIBIT II

                           ACKNOWLEDGMENT OF EXERCISE



The undersigned ____________________________________, hereby acknowledge receipt
of the "Notice of Exercise" from Alexandria Real Estate Equities, L.P., to
purchase __________ shares of the Stock of Acusphere, Inc., pursuant to the
terms of the Warrant Agreement, and further acknowledges that _________ shares
remain subject to purchase under the terms of the Warrant Agreement.



      COMPANY:                      ACUSPHERE, INC.



                                    By:__________________________________


                                    Title:_______________________________


                                    Date:________________________________
<PAGE>
                                      -11-

                                   EXHIBIT III

                                 TRANSFER NOTICE


(TO TRANSFER OR ASSIGN THE FOREGOING WARRANT AGREEMENT EXECUTE THIS FORM AND
SUPPLY REQUIRED INFORMATION. DO NOT USE THIS FORM TO PURCHASE SHARES.)

FOR VALUE RECEIVED, the foregoing Warrant Agreement and all rights evidenced
thereby are hereby transferred and assigned to

      _________________________________________________________________
                                 (PLEASE PRINT)

     WHOSE ADDRESS IS___________________________________________________

      __________________________________________________________________


                  DATED:      ____________________________________


                  HOLDER'S SIGNATURE:     _______________________________


                  HOLDER'S ADDRESS: _______________________________


                  ________________________________________________________


SIGNATURE GUARANTEED:   ____________________________________________


NOTE: The signature to this Transfer Notice must correspond with the name as it
      appears on the face of the Warrant Agreement, without alteration or
      enlargement or any change whatever. Officers of corporations and those
      acting in a fiduciary or other representative capacity should file proper
      evidence of authority to assign the foregoing Warrant Agreement.

<PAGE>
                                      -12-


                                   EXHIBIT IV

                                (INSERT CHARTER)