Document:

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                                                                  Exhibit 10.20

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933. 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.

                                WARRANT AGREEMENT

                  To Purchase Shares of the Preferred Stock of

                                 ACUSPHERE, INC.

               Dated as of October 19, 1998 (the "Effective Date")

     WHEREAS, Acusphere, Inc., a Delaware corporation (the "Company") has
entered into a Subordinated Loan And Security Agreement dated as of October 16,
1998, and related Subordinated Promissory Note(s) (collectively, the "Loans")
with Comdisco, Inc., a Delaware corporation (the "Warrantholder"); and

     WHEREAS, the Company desires to grant to Warrantholder, in consideration
for such Loans, the right to purchase shares of its Preferred Stock;

     NOW, THEREFORE, in consideration of the Warrantholder executing and
delivering such Loans 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 to and purchase, from the Company, 75,758 fully paid and
non-assessable shares of the Company's Series E Convertible Preferred Stock
("Preferred Stock") at a purchase price of $3.30 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.

     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

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purchased and shall execute the Notice of Exercise indicating the number of
shares which remain subject to future purchases, if any.

     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 will 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 Common Stock.

           B = the Exercise Price.

     As used herein, current fair market value of Common Stock shall mean with
respect to each share of Common Stock:

     (i) if the exercise is in connection with an initial public offering, and
     if the Company's Registration Statement relating to such public offering
     has been declared effective by the SEC, then 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, the fair market value 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 Common 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 Common 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 Common 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 Common 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

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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.

     (a) AUTHORIZATION AND RESERVATION OF SHARES. During the term of this
Warrant Agreement, the Company will 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.

     (b) REGISTRATION OR LISTING. If any shares of Preferred Stock required to
be reserved hereunder require registration with or approval of any governmental
authority under any Federal or State law (other than any registration under the
1933 Act, as then in effect, or any similar Federal statute then enforced, or
any state securities law, required by reason of any transfer involved in such
conversion), or listing on any domestic securities exchange, before such shares
may be issued upon conversion, the Company will, at its expense and as
expeditiously as possible, use its best efforts to cause such shares to be duly
registered, listed or approved for listing on such domestic securities exchange,
as the case may be.

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 when the Company is not 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 or other securities 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

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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; PREFERRED STOCK CONVERSION. 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, 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.

     (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 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) RIGHT TO PURCHASE ADDITIONAL STOCK. If the Company has not paid any
Subordinated Promissory Note(s) entered into pursuant to the Loan(s) in its
entirety by the Maturity Date (as defined in the applicable Subordinated
Promissory Note(s)), then for each additional month, or portion thereof,
thereafter that the outstanding principal is not paid, Warrantholder shall have
the right to purchase from the Company, at the Exercise Price (adjusted as set
forth herein), an additional number of shares of Preferred Stock which number
shall be determined by (i) multiplying the outstanding principal amount which is
due but unpaid by 1% and (ii) dividing the product thereof by the Exercise
Price.

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     (e) 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. 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, 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.

     (f) NOTICE OF ADJUSTMENTS. If: (i) the Company shall declare any dividend
or distribution upon its stock, whether in cash, property, stock or other
securities; (ii) the Company shall offer for subscription prorata to the holders
of any class of its Preferred or other convertible stock 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 or involuntary dissolution, liquidation or
winding up of the Company; 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 (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 a 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, (iii) the
method by which such adjustment was calculated, (iv) the Exercise Price, and (v)
the number of shares subject to purchase hereunder after 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.

     (g) TIMELY NOTICE. Failure to timely provide such notice required by
subsection (f) 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.

     (a) RESERVATION OF PREFERRED STOCK. The Preferred Stock issuable upon
exercise of the Warrantholder's rights has been duly and validly reserved and,
when issued in accordance with the provisions of this Warrant Agreement, will be
validly issued, fully paid and non-assessable, and will be free of any taxes,
liens, charges or encumbrances of any nature whatsoever; provided, however, that
the Preferred Stock issuable pursuant to this Warrant Agreement

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may be subject to restrictions on transfer under state and/or Federal securities
laws. The Company has made available to the Warrantholder true, correct and
complete copies of its Charter and Bylaws, as amended, and minutes of all Board
of Directors (including all committees of the Board of Directors, if any). The
issuance of certificates for shares of Preferred Stock upon exercise of the
Warrant Agreement shall be made without charge to the Warrantholder for any
issuance tax in respect thereof, or other cost incurred by the Company in
connection with such exercise and the related issuance of shares of Preferred
Stock. The Company shall not be required to pay any tax which may be payable in
respect of any transfer involved and the issuance and delivery of any
certificate in a name other than that of the Warrantholder.

     (b) DUE AUTHORITY. The execution and delivery by the Company of this
Warrant Agreement and the performance of all obligations of the Company
hereunder, including the issuance to Warrantholder of the right to acquire the
shares of Preferred Stock, have been duly authorized by all necessary corporate
action on the part of the Company, and the Leases and this Warrant Agreement are
not inconsistent with the Company's Charter or Bylaws, do not contravene any law
or governmental rule, regulation or order applicable to it, do not and will not
contravene any provision of, or constitute a default under, any indenture,
mortgage, contract or other instrument to which it is a party or by which it is
bound (except for any such provision or default as shall have been waived prior
to the execution hereof), and the Loans and this Warrant Agreement constitute
legal, valid and binding agreements of the Company, enforceable in accordance
with their respective terms.

     (c) CONSENTS AND APPROVALS. No consent or approval of, giving of notice to,
registration with, or taking of any other action in respect of any state,
Federal or other governmental authority or agency is required with respect to
the execution, delivery and performance by the Company of its obligations under
this Warrant Agreement, except for the filing of notices pursuant to Regulation
D under the 1933 Act and applicable state securities law, which filings will be
effective by the time required thereby.

     (d) ISSUED SECURITIES. All issued and outstanding shares of Common Stock,
Preferred Stock or any other securities of the Company have been duly authorized
and validly issued and are fully paid and nonassessable. All outstanding shares
of Common Stock, Preferred Stock and any other securities were issued in full
compliance with all Federal and state securities laws. In addition:

     (i) The authorized capital of the Company consists of (A) 17,000,000 shares
of Common Stock, of which 2,470,180 shares are issued and outstanding, and (B)
11,793,801 shares of preferred stock, of which 816,169 shares are designated
Series A Convertible Preferred Stock, all of which are issued and outstanding
and are convertible into 816,169 shares of Common Stock; and 2,315,625 shares
are designated Series B Convertible Preferred Stock, of which 2,265,625 shares
are issued and outstanding and are convertible into 2,265,625 shares of Common
Stock; and 3,980,140 shares are designated Series C Convertible Preferred Stock,
3,913,551 of which are issued and outstanding and are convertible into 3,913,551
shares of Common Stock; 3,498,882 shares are designated Series D Convertible
Preferred Stock, 3,405,624 of which are issued and outstanding and are
convertible into 3,405,624 shares of Common Stock, and 1,182,985 shares are
designated as Series E Convertible Preferred Stock, 757,577 of which are issued
and outstanding.

     (ii) The Company has reserved 1,385,458 shares of Common Stock for issuance
under its 1994 Stock Option Plan, under which 1,376,935 shares have been

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granted, of which 136,801 shares have been issued and of which 1,240,134 shares
are outstanding at an average price of $.21 per share. Except for this Warrant
Agreement and the Warrant Agreements dated as of May 1, 1995 (and reissued as of
August 19, 1998), August 21, 1996 (and reissued as of August 19, 1998), June 6,
1997 and February 28, 1998 between Comdisco, Inc. and the Company and the
Warrant Agreements originally dated as of August 21, 1996 (and reissued as of
August 19, 1998) and dated as of May 1, 1995 (and reissued as of August 19,
1998) between Gregory Stento and Comdisco, Inc., and except as set forth in this
paragraph (d), there are no other options, warrants, conversion privileges or
other rights presently outstanding to purchase or otherwise acquire any
authorized but unissued shares of the Company's capital stock or other
securities of the Company.

     (e) INSURANCE. The Company has in full force and effect insurance policies,
with extended coverage, insuring the Company and its property and business
against such losses and risks, and in such amounts, as are customary for
corporations engaged in a similar business and similarly situated and as
otherwise may be required pursuant to the terms of any other contract or
agreement.

     (f) EXEMPT TRANSACTION. Subject to the accuracy of the Warrantholder's
representations in Section 10 hereof, the issuance of the Preferred Stock upon
exercise of this Warrant will constitute a transaction exempt from the
registration requirements of Section 5 of the 1933 Act, in reliance upon Section
4(2) thereof and applicable state securities law.

     (g) COMPLIANCE WITH RULE 144. At the written request of the Warrantholder,
who proposes to sell Preferred Stock issuable upon the exercise of the Warrant
in compliance with Rule 144 promulgated by the Securities and Exchange
Commission, the Company shall furnish to the Warrantholder, within ten days
after receipt of such request, a written statement confirming the Company's
compliance with the filing requirements of the Securities and Exchange
Commission as set forth in such Rule, as such Rule may be amended from time to
time.

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) COMPANY'S RIGHT OF FIRST REFUSAL. The Warrantholder hereby agrees with
the Company as follows:

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     (i) Exercise of Right: If the Warrantholder or the Warrantholder's legal
representative (the "Transferor") desires to transfer all or any part of its
rights to acquire Preferred Stock hereunder, the Preferred Stock issued upon
exercise hereof or the Common Stock issuable upon conversion of the Preferred
Stock (collectively, the "Securities") to any person other than the Company (an
"Offeror"), the Transferor shall: (A) obtain in writing an irrevocable and
unconditional bona fide offer (the "Offer") for the purchase thereof from the
Offeror; and (B) give written notice (the "Notice") to the Company setting forth
the Warrantholder's desire to transfer such Securities, which Notice shall be
accompanied by a photocopy of the offer and shall set forth at least the name
and address of the Offeror and the price and terms of the bona fide offer. Upon
receipt of the Notice, the Company shall have an assignable option to purchase
any or all of such Securities (the "Company Option Securities") specified in the
Notice, such option to be exercisable by giving, within thirty (30) days after
receipt of the Notice, a written counter-notice to the Transferor. If the
Company elects to purchase any or all of such Company Option Securities, it
shall be obligated to purchase, and the Warrantholder shall be obligated to sell
to the Company, such Company Option Securities at the price and terms indicated
in the Offer within thirty (30) days from the date of delivery by the Company of
such counter-notice.

     (ii) Sale of Securities to Offeror: The Transferor may, for sixty (60) days
after the expiration of the thirty (30) day period during which the Company may
give the counter-notice, sell, pursuant to the terms of the offer, any or all of
such Company Option Securities not purchased or agreed to be purchased by the
Company or its assignee; provided, however, that the Transferor shall not sell
such Company Option Securities to the Offeror if the Offeror is a competitor of
the Company and the Company gives written notice to the Transferor, within
thirty (30) days of its receipt of the Notice, stating that the Transferor shall
not sell such Company Option Securities to such Offeror; and provided, further,
that prior to the sale of such Company Option Securities to the Offeror, the
Offeror shall execute an agreement with the Company pursuant to which the
Offeror agrees to be subject to the restrictions set forth in this Agreement. If
any or all of such Company Option Securities are not sold pursuant to an offer
within the time permitted above, the unsold Company Option Securities shall
remain subject to the terms of this Agreement.

     (iii) Failure to Deliver Company Option Securities: If the Transferor fails
or refuses to deliver on a timely basis duly endorsed certificates representing
Company Option Securities to be sold to the Company or its assignee pursuant to
this Agreement, the Company shall have the right to deposit the purchase price
for such Company Option Securities in a special account with any bank or trust
company in the Commonwealth of Massachusetts or State of Delaware, giving notice
of such deposit to the Transferor, whereupon such Company Option Securities
shall be deemed to have been purchased by the Company. All such monies shall be
held by the bank or trust company for the benefit of the Transferor. All monies
deposited with the bank or trust company remaining unclaimed for two years after
the date of deposit shall be repaid by the bank or trust company to the Company
on demand, and the Transferor shall thereafter look only to the Company for
payment. The Company may place a legend on any stock certificate delivered to
the Transferor reflecting the restrictions on transfer provided in this
Agreement.

     (iv) Expiration of Company's Right of First Refusal: The first refusal
rights of the Company set forth above shall remain in effect until such time, if
ever, as a distribution to the public is made of shares of the Company's Common
Stock pursuant to a registration statement filed under the Securities

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Act of 1933, as amended, or a successor statute, at which time the first refusal
rights of the Company set forth herein will automatically expire.

     (d) DISPOSITION OF WARRANTHOLDER'S RIGHTS. In no event will the
Warrantholder make a disposition of any of its rights to acquire Preferred Stock
or Preferred Stock issuable upon exercise of such rights or Common Stock
issuable upon conversion of such Preferred Stock 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 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.

     (e) 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.

     (f) 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 1933 Act, or file reports pursuant to Section 15(d), of the
Securities Exchange Act of 1934 (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.

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

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in all of the Warrants exceed three (3) transfers. 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 II (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.

     (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 attorney's 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
Commonwealth of Massachusetts.

     (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
6111 North River Road, Rosemont, Illinois 60018, attention: James Labe, Venture
Leasing Director, cc: Legal Department, (and/or, if by facsimile, (708)
518-5465) and (ii) to the company at 38 Sidney Street, Cambridge, Massachusetts
02139, (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.

                                       10
<PAGE>   11

     (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.

     (k) ADDITIONAL DOCUMENTS. The Company, upon execution of this Warrant
Agreement, shall provide the Warrantholder with certified resolutions
authorizing the execution, delivery and performance of this Warrant Agreement
and the Loans referred to herein.

     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 C. Oberg
                                            ------------------------------------

                                        Title: President and CEO
                                               ---------------------------------

                                        Warrantholder: COMDISCO, INC.

                                        By: /s/ James P. Labe
                                            ------------------------------------
                                                JAMES P. LABE, PRESIDENT

                                        Title: Comdisco Ventures Division
                                               ---------------------------------

                                       11
<PAGE>   12

                                    EXHIBIT I

                               NOTICE OF EXERCISE

To:_________________________________________

(1)  The undersigned Warrantholder hereby elects to purchase _____ shares of the
     Preferred Stock of ____________________, pursuant to the terms of the
     Warrant Agreement dated the ______ day of _____________________, 19__ (the
     "Warrant Agreement") between __________________ and the Warrantholder, and
     tenders herewith payment of the purchase price for such shares in full,
     together with all applicable transfer taxes, if any.

(2)  In exercising its rights to purchase the Preferred 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
     Preferred Stock in the name of the undersigned or in such other name as is
     specified below.

____________________________________________
(Name)

____________________________________________
(Address)

Warrantholder:   COMDISCO, INC.

By:_________________________________________

Title:______________________________________

Date:_______________________________________

                                       12
<PAGE>   13

                           ACKNOWLEDGMENT OF EXERCISE

     The undersigned ____________________________, hereby acknowledge receipt
of the "Notice of Exercise" from Comdisco, Inc., to purchase ________ shares of
the Preferred Stock of _______________________, 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:

                                        By:_____________________________________

                                        Title:__________________________________

                                        Date:___________________________________

                                       13
<PAGE>   14

                                   EXHIBIT II

                                 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_______________________________________________________

                    Holders Signature___________________________________________

                    Holders 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.

                                       14<PAGE>   1
                                                                   Exhibit 10.21

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933. 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.

                                WARRANT AGREEMENT

          TO PURCHASE SHARES OF SERIES E CONVERTIBLE PREFERRED STOCK OF

                                 ACUSPHERE, INC.

               DATED AS OF JANUARY 5, 2000 (THE "EFFECTIVE DATE")

     WHEREAS, Acusphere, Inc., a Delaware corporation (the "COMPANY") has
entered into a Master Lease Agreement dated as of May 1, 1995, Equipment
Schedules No. VL-8 and No. VL-9 dated November 24, 1999, and related Schedules
(the "LEASES") with Comdisco, Inc., a Delaware corporation (the
"WARRANTHOLDER"); and

     WHEREAS, the Company desires to grant to Warrantholder, in consideration
for such Leases, the right to purchase shares of its Series E Preferred Stock;

     NOW, THEREFORE, in consideration of the Warrantholder executing and
delivering such Leases 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 to and purchase, from the Company, 13,637 fully paid and
non-assessable shares of the Company's Series E Convertible Preferred Stock
("PREFERRED STOCK") at a purchase price of $3.30 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 (i) ten (10) years
from the Effective Date, or (ii) five (5) years after the effective date of the
Company's initial public offering, whichever is earlier.

3.   EXERCISE OF THE PURCHASE RIGHTS.

     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 will 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 Common Stock.

           B = the Exercise Price.

     As used herein, current fair market value of Common Stock shall mean with
respect to each share of Common Stock:

     (i) if the exercise is in connection with an initial public offering, and
     if the Company's Registration Statement relating to such public offering
     has been declared effective by the SEC, then 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, the fair market value 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 Common 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 Common 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 Common 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 Common 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.

                                      -2-
<PAGE>   3

4.   AUTHORIZATION AND RESERVATION OF SHARES.

     During the term of this Warrant Agreement, the Company will 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 when the Company is not 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 or other securities 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; PREFERRED STOCK CONVERSION. 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, this Warrant Agreement shall
no longer be exercisable for Preferred Stock, but shall represent the right to
acquire that number of

                                      -3-
<PAGE>   4

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.

     (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 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) 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 II (the "CHARTER"). The
Company shall promptly provide the Warrantholder with any restatement,
amendment, modification or waiver of the Charter. 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, 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.

     (f) NOTICE OF ADJUSTMENTS. If: (i) the Company shall declare any dividend
or distribution upon its stock, whether in cash, property, stock or other
securities; (ii) the Company shall offer for subscription prorata to the holders
of any class of its Preferred or other convertible stock 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 or involuntary dissolution, liquidation or
winding up of the Company; 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 (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 a 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, (iii) the
method by which such adjustment was calculated, (iv) the Exercise Price, and (v)
the number of shares subject to purchase hereunder after 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.

                                      -4-
<PAGE>   5

     (g) TIMELY NOTICE. Failure to timely provide such notice required by
subsection (f) 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.   REPRESENTATION, WARRANTIES AND COVENANTS OF THE COMPANY.

     (a) RESERVATION OF PREFERRED STOCK. The Preferred Stock issuable upon
exercise of the Warrantholder's rights has been duly and validly reserved and,
when issued in accordance with the provisions of this Warrant Agreement, will be
validly issued, fully paid and non-assessable, and will be free of any taxes,
liens, charges or encumbrances of any nature whatsoever; provided, however, that
the Preferred Stock issuable pursuant to this Warrant Agreement may be subject
to restrictions on transfer under state and/or federal securities laws. The
Company has made available to the Warrantholder true, correct and complete
copies of its Charter and Bylaws, as amended, and minutes of all Board of
Directors (including all committees of the Board of Directors, if any). The
issuance of certificates for shares of Preferred Stock upon exercise of the
Warrant Agreement shall be made without charge to the Warrantholder for any
issuance tax in respect thereof, or other cost incurred by the Company in
connection with such exercise and the related Issuance of shares of Preferred
Stock. The Company shall not be required to pay any tax which may be payable in
respect of any transfer involved and the issuance and delivery of any
certificate in a name other than that of the Warrantholder.

     (b) DUE AUTHORITY. The execution and delivery by the Company of this
Warrant Agreement and the performance of all obligations of the Company
hereunder, including the issuance to Warrantholder of the right to acquire the
shares of Preferred Stock, have been duly authorized by all necessary corporate
action on the part of the Company, and the Leases and this Warrant Agreement are
not inconsistent with the Company's Charter or Bylaws, do not contravene any law
or governmental rule, regulation or order applicable to it, do not and will not
contravene any provision of, or constitute a default under, any indenture,
mortgage, contract or other instrument to which it is a party or by which it is
bound (except for any such provision or default as shall have been waived prior
to the execution hereof), and the Leases and this Warrant Agreement constitute
legal, valid and binding agreements of the Company, enforceable in accordance
with their respective terms.

     (c) CONSENTS AND APPROVALS. No consent or approval of, giving of notice to,
registration with, or taking of any other action in respect of any state,
Federal or other governmental authority or agency is required with respect to
the execution, delivery and performance by the Company of its obligations under
this Warrant Agreement, except for the filing of notices pursuant to Regulation
D under the 1933 Act and applicable state securities law, which filings will be
effective by the time required thereby.

     (d) ISSUED SECURITIES. All issued and outstanding shares of Common Stock,
Preferred Stock or any other securities of the Company have been duly authorized
and validly issued and are fully paid and nonassessable. All outstanding shares
of Common Stock, Preferred Stock and any other securities were issued in full
compliance with all Federal and state securities laws. In addition as of the
Effective Date:

     (i) The authorized capital of the Company consists of (A) 17,000,000 shares
of Common Stock, of which 2,564,561 shares are issued and outstanding, and
(B) 11,843,801 shares of preferred stock, of which 816,169 shares are designated
Series A Convertible Preferred Stock, all of which are issued and outstanding
and are convertible into 816,169 shares of Common Stock; and 2,315,625 shares
are designated Series B Convertible Preferred Stock, of which 2,265,625 shares
are issued and outstanding and are convertible into 2,265,625 shares of Common
Stock; 3,980,140 shares are designated Series C Convertible Preferred Stock,
3,913,551 of which are issued and outstanding and are convertible into 3,913,551
shares of Common Stock; 3,498,882 shares are designated Series D Convertible
Preferred Stock, 3,405,624 of which are issued and outstanding and are
convertible into 3,405,624

                                      -5-
<PAGE>   6

shares of Common Stock., and 1,232,985 shares are designated as Series E
Convertible Preferred stock, 757,577 of which are issued and outstanding.

     (ii) The Company has reserved 1,985,458 shares of Common Stock for Issuance
under its 1994 Stock Plan, under which options to purchase 1,427,635 stares have
been granted, of which 147,992 shares have been issued and of which options to
purchase 1,240,134 shares are outstanding at an average price of $21 per share.
Except for this Warrant Agreement and the Warrant Agreements dated as of May 1,
1995 (and reissued as of August 19,1998), August 21, 1996 (and reissued as of
August 19, 1998), June 6, 1997, February 26, 1998, October 16, 1999 and October
19, 1998 between Comdisco, Inc. and the Company and the Warrant Agreements
originally dated as of August 21, 1996 (and reissued as of August 19,1998) and
dated as of May 1, 1995 (and reissued as of August 19, 1998) between Gregory
Stento and the Company, and except as set forth in this paragraph (d), there are
no other options, warrants, conversion privileges or other rights presently
outstanding to purchase or otherwise acquire any authorized but unissued shares
of the Company's capital stock or other securities of the Company.

     (e) Insurance. The Company has in full force and effect insurance policies,
with extended coverage, insuring the Company and its property and business
against such losses and risks, and in such amounts, as are customary for
corporations engaged in a similar business and similarly situated and as
otherwise may be required pursuant to the terms of any other contract or
agreement.

     (f) Exempt Transaction. Subject to the accuracy of the Warrantholder's
representations in Section 10 hereof, the issuance of the Preferred Stock upon
exercise of this Warrant will constitute a transaction exempt from the
registration requirements of Section 5 of the 1933 Act, in reliance upon Section
4(2) thereof and applicable state securities law.

     (g) Compliance with Rule 144. At the written request of the Warrantholder,
who proposes to sell Preferred Stock issuable upon the exercise of the Warrant
in compliance with Rule 144 promulgated by the Securities and Exchange
Commission, the Company shall furnish to the Warrantholder, within ten days
after receipt of such request, a written statement confirming the Company's
compliance with the filing requirements of the Securities and Exchange
Commission as set forth in such Rule, as such Rule may be amended from time to
time.

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) Company's Right of First Refusal. The Warrantholder hereby agrees with
the Company as follows:

     (i) Exercise of Right: If the Warrantholder or the Warrantholder's legal
representative (the "Transferor") desires to transfer all or any part of its
rights to acquire Preferred Stock hereunder, the Preferred Stock

                                      -6-
<PAGE>   7

issued upon exercise hereof or the Common Stock issuable upon conversion of the
Preferred Stock (collectively, the "Securities") to any person other than the
Company (an "OFFEROR"), the Transferor shall: (A) obtain in writing an
irrevocable and unconditional bona fide offer (the "Offer") for the purchase
thereof from the Offeror; and (B) give written notice (the "Notice") to the
Company setting forth the Warrantholder's desire to transfer such Securities,
which Notice shall be accompanied by a photocopy of the Offer and shall set
forth at least the name and address of the Offeror and the price and terms of
the bona fide offer. Upon receipt of the Notice, the Company shall have an
assignable option to purchase any or all of such Securities (the "Company Option
Securities") specified in the Notice, such option to be exercisable by giving,
within thirty (30) days after receipt of the Notice, a written counter-notice to
the Transferor. If the Company elects to purchase any or all of such Company
Option Securities, it shall be obligated to purchase, and the Warrantholder
shall be obligated to sell to the Company, such Company Option Securities at the
price and terms indicated in the Offer within thirty (30) days from the date of
delivery by the Company of such counter-notice.

     (ii) Sale of Securities to Offeror: The Transferor may, for sixty (60) days
after the expiration of the thirty (30) day period during which the Company may
give the counter-notice, sell, pursuant to the terms of the Offer, any or all of
such Company Option Securities not purchased or agreed to be purchased by the
Company or its assignee; provided, however, that the Transferor shall not sell
such Company Option Securities to the Offeror if the Offeror is a competitor of
the Company and the Company gives written notice to the Transferor, within
thirty (30) days of its receipt of the Notice, stating that the Transferor shall
not sell such Company Option Securities to such Offeror; and provided, further,
that prior to the sale of such Company Option Securities to the Offeror, the
Offeror shall execute an agreement with the Company pursuant to which the
Offeror agrees to be subject to the restrictions set forth in this Agreement. If
any or all of such Company Option Securities are not sold pursuant to an Offer
within the time permitted above, the unsold Company Option Securities shall
remain subject to the terms of this Agreement.

     (iii) Failure to Deliver Company Option Securities: If the Transferor fails
or refuses to deliver on a timely basis duly endorsed certificates representing
Company Option Securities to be sold to the Company or its assignee pursuant to
this Agreement, the Company shall have the right to deposit the purchase price
for such Company Option Securities in a special account with any bank or trust
company in the Commonwealth of Massachusetts or State of Delaware, giving notice
of such deposit to the Transferor, whereupon such Company Option Securities
shall be deemed to have been purchased by the Company. All such monies shall be
held by the bank or trust company for the benefit of the Transferor. All monies
deposited with the bank or trust company remaining unclaimed for two years after
the date of deposit shall be repaid by the bank or trust company to the Company
on demand, and the Transferor shall thereafter look only to the Company for
payment. The Company may place a legend on any stock certificate delivered to
the Transferor reflecting the restrictions on transfer provided in this
Agreement.

     (iv) Expiration of Company's Right of First Refusal: The first refusal
rights of the Company set forth above shall remain in effect until such time, if
ever, as a distribution to the public is made of shares of the Company's Common
Stock pursuant to a registration statement filed under the Securities Act of
1933, as amended, or a successor statute, at which time the first refusal rights
of the Company set forth herein will automatically expire.

         (d) Disposition of Warrantholder's Rights. In no event will the
Warrantholder make a disposition of any of its rights to acquire Preferred Stock
or Preferred Stock issuable upon exercise of such rights or Common Stock
issuable upon conversion of such Preferred Stock 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 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

                                      -7-
<PAGE>   8

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.

     (e) 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.

     (f) 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 1933 Act, or file reports pursuant to Section 15(d), of the
Securities Exchange Act of 1934 (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.

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. 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.

     (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
Commonwealth of Massachusetts.

     (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) Notice. 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

                                      -8-
<PAGE>   9

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 6111 North River Road, Rosemont, Illinois 60018, attention: Jill Hanses,
Venture Leasing, cc: Legal Department, (and/or, if by facsimile, (708) 518-5465
and (ii) to the Company at 38 Sidney Street, Cambridge, Massachusetts 02139,
(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.

     (k) Additional Documents. The Company, upon execution of this Warrant
Agreement, shall provide the Warrantholder with certified resolutions
authorizing the execution, delivery and performance of this Warrant Agreement
and the Leases referred to herein.

                            [SIGNATURE PAGE FOLLOWS]

                                      -9-
<PAGE>   10

     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 C. Oberg
                                            ------------------------------------

                                        Title: President and CEO
                                               ---------------------------------

                                        Warrantholder: COMDISCO, INC.

                                        By: /s/ James Labe
                                            ------------------------------------

                                                JAMES LABE, PRESIDENT
                                        Title: COMDISCO VENTURES DIVISION
                                               ---------------------------------

                                      -10-
<PAGE>   11

                                    EXHIBIT 1

                               NOTICE OF EXERCISE

To:
    --------------------------------

(1)  The undersigned Warrantholder hereby elects to purchase ---- shares of the
     Series E Preferred Stock of Acusphere, Inc. (the "COMPANY"), pursuant to
     the terms of the Warrant Agreement dated the 24th day of November, 1999
     (the "Warrant Agreement") between the Company and the Warrantholder, and
     tenders herewith payment of the purchase price for such shares in full,
     together with all applicable transfer taxes, if any.

(2)  In exercising its rights to purchase the Preferred Stock of the Company,
     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
     Preferred Stock in the name of the undersigned or in such other name as is
     specified below.

------------------------------------------
(Name)

------------------------------------------
(Address)

Warrantholder: COMDISCO, INC.

By:
   -----------------------------------------

Title:
      --------------------------------------

Date:
     ---------------------------------------

                                      -11-
<PAGE>   12

                           ACKNOWLEDGMENT OF EXERCISE

     The undersigned _______________________, hereby acknowledge receipt of the
"NOTICE OF EXERCISE" from Comdisco, Inc., to purchase ____ shares of the Series
E Convertible Preferred 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: __________________________________

                                      -12-
<PAGE>   13

                                   EXHIBIT II

                               (CHARTER DOCUMENTS)

                                      -13-
<PAGE>   14

                                   EXHIBIT III

                                 TRANSFER NOTICE

(To transfer or assign the foregoing Warrant Agreement execute this form and
supply required information. To 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.

                                      -14-

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