<PAGE>

                                                                   Exhibit 10.26

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED OR ANY STATE SECURITIES LAWS. NO SALE, OFFER FOR SALE, PLEDGE,
HYPOTHECATION OR DISPOSITION MAY BE EFFECTED WITHOUT (i) EFFECTIVE REGISTRATION
STATEMENTS RELATED THERETO, (ii) AN OPINION OF COUNSEL OR OTHER EVIDENCE,
REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATIONS ARE NOT
REQUIRED, (iii) RECEIPT OF A NO-ACTION LETTER ADDRESSED TO THE HOLDER HEREOF
FROM THE APPROPRIATE GOVERNMENTAL AUTHORITIES, OR (iv) OTHERWISE COMPLYING WITH
THE PROVISIONS OF SECTION 7 OF THIS WARRANT.

                                 ACUSPHERE, INC.

                        WARRANT TO PURCHASE 68,421 SHARES
                           OF SERIES F PREFERRED STOCK

      THIS CERTIFIES THAT, for value received, VENTURE LENDING & LEASING III,
LLC and its assignees are entitled to subscribe for and purchase 68,421 shares
of the fully paid and nonassessable Series F Non-Voting Preferred Stock (as
adjusted pursuant to Section 4 hereof, the "Shares") of ACUSPHERE, INC., a
Delaware corporation (the "Company"), at the price of $4.75 per share (such
price and such other price as shall result, from time to time, from the
adjustments specified in Section 4 hereof is herein referred to as the "Warrant
Price"), subject to the provisions and upon the terms and conditions hereinafter
set forth.

      As used herein, (a) the term "Series Preferred" shall mean the Company's
presently authorized Series F Non-Voting Preferred Stock, par value $.01 per
share, and any stock into or for which such Series F Non-Voting Preferred Stock
may hereafter be converted or exchanged, and after the conversion of the Series
F Non-Voting Preferred Stock to the Company's Common Stock, par value $.01 per
share, shall mean the Company's Common Stock, (b) the term "Date of Grant" shall
mean September 27, 2001, and (c) the term "Other Warrants" shall mean any other
warrants issued by the Company in connection with the transaction with respect
to which this Warrant was issued, and any warrant issued upon transfer or
partial exercise of or in lieu of this Warrant. The term "Warrant" as used
herein shall be deemed to include Other Warrants unless the context clearly
requires otherwise.

      1.    Term. The purchase right represented by this Warrant is exercisable,
in whole or in part, at any time and from time to time from the Date of Grant
through the later of (i) ten (10) years after the Date of Grant or (ii) five (5)
years after the closing of the Company's initial public offering of its Common
Stock ("IPO") effected pursuant to a Registration Statement on Form S-1 (or its
successor) filed under the Securities Act of 1933, as amended (the "Act").

      2.    Method of Exercise; Payment; Issuance of New Warrant. Subject to
Section 1 hereof, the purchase right represented by this Warrant may be
exercised by the holder hereof, in whole or in part and from time to time, at
the election of the holder hereof, by (a) the surrender of this Warrant (with
the notice of exercise substantially in the form attached hereto as Exhibit A-1
duly completed
<PAGE>
and executed) at the principal office of the Company and by the payment to the
Company, by certified or bank check, or by wire transfer to an account
designated by the Company (a "Wire Transfer") of an amount equal to the then
applicable Warrant Price multiplied by the number of Shares then being
purchased; (b) if in connection with a registered public offering of the
Company's securities, in connection with which the holder of this Warrant has
duly exercised the registration rights of such holder in that certain Eighth
Amended and Restated Investors' Rights Agreement, by and among the Company and
the Investors named therein, dated as of June 1, 2001, as amended and in effect
from time to time (the "Registration Rights Agreement"), the surrender of this
Warrant (with the notice of exercise form attached hereto as Exhibit A-2 duly
completed and executed) at the principal office of the Company together with
notice of arrangements reasonably satisfactory to the Company for payment to the
Company either by certified or bank check or by Wire Transfer from the proceeds
of the sale of shares to be sold by the holder in such public offering of an
amount equal to the then applicable Warrant Price per share multiplied by the
number of Shares then being purchased; or (c) exercise of the "net issuance"
right provided for in Section 10.2 hereof. The person or persons in whose
name(s) any certificate(s) representing shares of Series Preferred shall be
issuable upon exercise of this Warrant shall be deemed to have become the
holder(s) of record of, and shall be treated for all purposes as the record
holder(s) of, the shares represented thereby (and such shares shall be deemed to
have been issued) immediately prior to the close of business on the date or
dates upon which this Warrant is exercised. In the event of any exercise of the
rights represented by this Warrant, certificates for the shares of stock so
purchased shall be delivered to the holder hereof as soon as possible and in any
event within thirty (30) days after such exercise and, unless this Warrant has
been fully exercised or expired, a new Warrant representing the portion of the
Shares, if any, with respect to which this Warrant shall not then have been
exercised shall also be issued to the holder hereof as soon as possible and in
any event within such thirty-day period; provided, however, at such time as the
Company is subject to the reporting requirements of the Securities Exchange Act
of 1934, as amended, if requested by the holder of this Warrant, the Company
shall cause its transfer agent to deliver the certificate representing Shares
issued upon exercise of this Warrant to a broker or other person (as directed by
the holder exercising this Warrant) within a reasonable time, not exceeding
three (3) business days after such exercise.

      3.    Stock Fully Paid; Reservation of Shares. All Shares that may be
issued upon the exercise of the rights represented by this Warrant will, upon
issuance pursuant to the terms and conditions herein, be fully paid and
nonassessable, and free from all taxes, liens and charges with respect to the
issue thereof. During the period within which the rights represented by this
Warrant may be exercised, the Company will at all times have authorized, and
reserved for the purpose of the issue upon exercise of the purchase rights
evidenced by this Warrant, a sufficient number of shares of its Series Preferred
to provide for the exercise of the rights represented by this Warrant and a
sufficient number of shares of its Common Stock to provide for the conversion of
the Series Preferred into Common Stock.

      4.    Adjustment of Warrant Price and Number of Shares. The number and
kind of securities purchasable upon the exercise of this Warrant and the Warrant
Price shall be subject to adjustment from time to time upon the occurrence of
certain events, as follows:


                                     - 2 -
<PAGE>
            (a)   Reclassification or Merger. In case of any reclassification or
change of securities of the class issuable upon exercise of this Warrant (other
than a change in par value, or from par value to no par value, or from no par
value to par value, or as a result of a subdivision or combination), or in case
of any merger of the Company with or into another corporation (other than a
merger with another corporation in which the Company is the acquiring and the
surviving corporation and which does not result in any reclassification or
change of outstanding securities issuable upon exercise of this Warrant), or in
case of any sale of all or substantially all of the assets of the Company, the
Company, or such successor or purchasing corporation, as the case may be, shall
duly execute and deliver to the holder of this Warrant a new Warrant (in form
and substance consistent with the terms of this Warrant and reasonably
satisfactory to the holder of this Warrant), or the Company shall make
appropriate provision without the issuance of a new Warrant, so that the holder
of this Warrant shall have the right to receive, at a total purchase price not
to exceed that payable upon the exercise of the unexercised portion of this
Warrant, and in lieu of the shares of Series Preferred theretofore issuable upon
exercise of this Warrant, (i) the kind and amount of shares of stock, other
securities, money and property receivable upon such reclassification, change,
merger or sale by a holder of the number of shares of Series Preferred then
purchasable under this Warrant, or (ii) in the case of such a merger or sale in
which the consideration paid consists all or in part of assets other than
securities of the successor or purchasing corporation, at the option of the
Holder of this Warrant and with the consent of the Company, the securities of
the successor or purchasing corporation having a value at the time of the
transaction equivalent to the valuation of the Series Preferred at the time of
the transaction. Any new Warrant shall provide for adjustments that shall be as
nearly equivalent as may be practicable to the adjustments provided for in this
Section 4. The provisions of this subparagraph (a) shall similarly apply to
successive reclassifications, changes, mergers and transfers.

            (b)   Subdivision or Combination of Shares. If the Company at any
time while this Warrant remains outstanding and unexpired shall subdivide or
combine its outstanding shares of Series Preferred, the Warrant Price shall be
proportionately decreased and the number of Shares issuable hereunder shall be
proportionately increased in the case of a subdivision and the Warrant Price
shall be proportionately increased and the number of Shares issuable hereunder
shall be proportionately decreased in the case of a combination.


            (c)   Stock Dividends and Other Distributions. If the Company at any
time while this Warrant is outstanding and unexpired shall (i) pay a dividend
with respect to Series Preferred payable in Series Preferred, then the Warrant
Price shall be adjusted, from and after the date of determination of
shareholders entitled to receive such dividend or distribution, to that price
determined by multiplying the Warrant Price in effect immediately prior to such
date of determination by a fraction (A) the numerator of which shall be the
total number of shares of Series Preferred outstanding immediately prior to such
dividend or distribution, and (B) the denominator of which shall be the total
number of shares of Series Preferred outstanding immediately after such dividend
or distribution; or (ii) make any other distribution with respect to Series
Preferred (except any distribution specifically provided for in Sections 4(a)
and 4(b) and excluding any cumulative dividends declared and paid under Section
1.3 of the Amended and Restated Certificate of Incorporation of the Company, as
amended from time to time), then, in each such case, provision shall be made by
the Company such that the holder of this Warrant shall receive upon exercise of
this


                                     - 3 -
<PAGE>
Warrant a proportionate share of any such dividend or distribution as though it
were the holder of the Series Preferred (or Common Stock issuable upon
conversion thereof) as of the record date fixed for the determination of the
shareholders of the Company entitled to receive such dividend or distribution.

            (d)   Adjustment of Number of Shares. Upon each adjustment in the
Warrant Price, the number of Shares of Series Preferred purchasable hereunder
shall be adjusted, to the nearest whole share, to the product obtained by
multiplying the number of Shares purchasable immediately prior to such
adjustment in the Warrant Price by a fraction, the numerator of which shall be
the Warrant Price immediately prior to such adjustment and the denominator of
which shall be the Warrant Price immediately thereafter.

            (e)   Antidilution Rights. The other antidilution rights applicable
to the Shares of Series Preferred purchasable hereunder are set forth in the
Company's Amended and Restated Certificate of Incorporation, as amended through
the Date of Grant, a true and complete copy of which is attached hereto as
Exhibit B (the "Charter"). Such antidilution rights shall not be restated,
amended, modified or waived in any manner that treats the holder hereof
differently than other holders of the Series F Non-Voting Preferred Stock. The
Company shall promptly provide the holder hereof with any restatement,
amendment, modification or waiver of the Charter promptly after the same has
been made.

      5.    Notice of Adjustments. Whenever the Warrant Price or the number of
Shares purchasable hereunder shall be adjusted pursuant to Section 4 hereof, the
Company shall make a certificate signed by its principal financial officer
setting forth, in reasonable detail, the event requiring the adjustment, the
amount of the adjustment, the method by which such adjustment was calculated,
and the Warrant Price and the number of Shares purchasable hereunder after
giving effect to such adjustment, and shall cause copies of such certificate to
be mailed (without regard to Section 13 hereof, by first class mail, postage
prepaid) to the holder of this Warrant. In addition, whenever the conversion
price or conversion ratio of the Series Preferred shall be adjusted, the Company
shall make a certificate signed by its principal financial officer setting
forth, in reasonable detail, the event requiring the adjustment, the amount of
the adjustment, the method by which such adjustment was calculated, and the
conversion price or ratio of the Series Preferred after giving effect to such
adjustment, and shall cause copies of such certificate to be mailed (without
regard to Section 13 hereof, by first class mail, postage prepaid) to the holder
of this Warrant.

      6.    Fractional Shares. No fractional shares of Series Preferred or scrip
representing fractional shares of Series Preferred will be issued in connection
with any exercise hereunder, but in lieu of such fractional shares or scrip the
Company shall make a cash payment therefor based on the fair market value of the
Series Preferred on the date of exercise as reasonably determined in good faith
by the Company's Board of Directors.

      7.    Compliance with Act; Disposition of Warrant or Shares of Series
Preferred.


            (a)   Compliance with Act. The holder of this Warrant, by acceptance
hereof, agrees that this Warrant, and the shares of Series Preferred to be
issued upon exercise hereof and any


                                     - 4 -
<PAGE>
Common Stock issued upon conversion thereof are being acquired for investment
and that such holder will not offer, sell, pledge, hypothecate or otherwise
dispose of this Warrant, or any shares of Series Preferred to be issued upon
exercise hereof or any Common Stock issued upon conversion thereof except under
circumstances which will not result in a violation of the Act or any applicable
state securities laws, and in any event, without an effective registration
statement related thereto, an opinion of counsel or other evidence satisfactory
to the Company that such registration is not required, or receipt of a no-action
letter addressed to the holder hereof from the appropriate governmental
authorities. Upon exercise of this Warrant, unless the Shares being acquired are
registered under the Act and any applicable state securities laws or an
exemption from such registration is available, the holder hereof shall confirm
in writing that the shares of Series Preferred so purchased (and any shares of
Common Stock issued upon conversion thereof) are being acquired for investment
and not with a view toward distribution or resale in violation of the Act and
shall confirm such other matters related thereto as may be reasonably requested
by the Company. This Warrant and all shares of Series Preferred issued upon
exercise of this Warrant and all shares of Common Stock issued upon conversion
thereof (unless registered under the Act and any applicable state securities
laws) shall be stamped or imprinted with a legend in substantially the following
form:

"THE SECURITIES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS. NO SALE, OFFER FOR SALE,
PLEDGE, HYPOTHECATION OR DISPOSITION MAY BE EFFECTED WITHOUT (i) EFFECTIVE
REGISTRATION STATEMENTS RELATED THERETO, (ii) AN OPINION OF COUNSEL OR OTHER
EVIDENCE, REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATIONS ARE
NOT REQUIRED, (iii) RECEIPT OF A NO-ACTION LETTER ADDRESSED TO THE HOLDER HEREOF
FROM THE APPROPRIATE GOVERNMENTAL AUTHORITIES, OR (iv) OTHERWISE COMPLYING WITH
THE PROVISIONS OF SECTION 7 OF THE WARRANT UNDER WHICH THESE SECURITIES WERE
ISSUED, DIRECTLY OR INDIRECTLY."

      Said legend shall be removed by the Company, upon the request of a holder,
at such time as the restrictions on the transfer of the applicable security
shall have terminated. In addition, in connection with the issuance of this
Warrant, the holder specifically represents to the Company by acceptance of this
Warrant as follows:

                  (1)   The holder is aware of the Company's business affairs
and financial condition, and has acquired information about the Company
sufficient to reach an informed and knowledgeable decision to acquire this
Warrant, or any shares of Series Preferred to be issued upon exercise hereof or
any Common Stock issued upon conversion thereof. The holder is acquiring this
Warrant, or any shares of Series Preferred to be issued upon exercise hereof or
any Common Stock issued upon conversion thereof, for its own account for
investment purposes only and not with a view to, or for the resale in connection
with, any "distribution" thereof in violation of the Act.

                  (2)   The holder understands that this Warrant has not been
registered under the Act in reliance upon a specific exemption therefrom, which
exemption depends upon, among other things, the bona fide nature of the holder's
investment intent as expressed herein.


                                     - 5 -
<PAGE>
                  (3)   The holder further understands that this Warrant and any
shares of Series Preferred to be issued upon exercise hereof or any Common Stock
issued upon conversion thereof must be held indefinitely unless subsequently
registered under the Act and qualified under any applicable state securities
laws, or unless exemptions from registration and qualification are otherwise
available. The holder is aware of the provisions of Rule 144, promulgated under
the Act.

                  (4)   The holder is an "accredited investor" as such term is
defined in Rule 501 of Regulation D promulgated under the Act.

            (b)   Disposition of Warrant or Shares. With respect to any offer,
sale or other disposition of this Warrant,or any shares of Series Preferred to
be issued upon exercise hereof or any Common Stock issued upon conversion
thereof, acquired pursuant to the exercise of this Warrant prior to registration
of such Warrant or shares, the holder hereof agrees to give written notice to
the Company prior thereto, describing briefly the manner thereof, together with
a written opinion of such holder's counsel, or other evidence, if reasonably
satisfactory to the Company, to the effect that such offer, sale or other
disposition may be effected without registration or qualification (under the Act
as then in effect or any federal or state securities law then in effect) of this
Warrant or such shares of Series Preferred or Common Stock and indicating
whether or not under the Act certificates for this Warrant or such shares of
Series Preferred or Common Stock to be sold or otherwise disposed of require any
restrictive legend as to applicable restrictions on transferability in order to
ensure compliance with such law. Upon receiving such written notice and
reasonably satisfactory opinion or other evidence, the Company, as promptly as
practicable but no later than fifteen (15) days after receipt of the written
notice, shall notify such holder that such holder may sell or otherwise dispose
of this Warrant or such shares of Series Preferred or Common Stock, all in
accordance with the terms of the notice delivered to the Company and this
Warrant. If a determination has been made pursuant to this Section 7(b) that the
opinion of counsel for the holder or other evidence is not reasonably
satisfactory to the Company, the Company shall so notify the holder promptly
with details thereof after such determination has been made. Notwithstanding the
foregoing, this Warrant or such shares of Series Preferred or Common Stock may,
as to such federal laws, be offered, sold or otherwise disposed of in accordance
with Rule 144 or 144A under the Act, provided that the Company shall have been
furnished with such information as the Company may reasonably request to provide
a reasonable assurance that the provisions of Rule 144 or 144A have been
satisfied. Each certificate representing this Warrant or the shares of Series
Preferred or Common Stock thus transferred (except a transfer pursuant to Rule
144 or 144A) shall bear a legend as to the applicable restrictions on
transferability in order to ensure compliance with such laws, unless in the
aforesaid opinion of counsel for the holder, such legend is not required in
order to ensure compliance with such laws. The Company may issue stop transfer
instructions to its transfer agent in connection with such restrictions.

            (c)   Applicability of Restrictions. Subject to compliance with
federal and state securities laws, neither any restrictions of any legend
described in this Warrant nor the requirements of Section 7(b) above shall apply
to any transfer of, or grant of a security interest in, this Warrant (or the
Series Preferred or Common Stock obtainable upon exercise thereof) or any part
hereof (i) to a partner of the holder if the holder is a partnership or to a
member of the holder if the holder is a limited liability company, (ii) to a
partnership of which the holder is a partner or to a limited liability


                                     - 6 -
<PAGE>
company of which the holder is a member, or (iii) to any affiliate of the holder
if the holder is a corporation; provided, however, in any such transfer, if
applicable, the transferee shall agree in writing to be bound by the terms of
this Warrant as if an original holder hereof.

      8.    Rights as Shareholders; Information. No holder of this Warrant, as
such, shall be entitled to vote or receive dividends or be deemed the holder of
Series Preferred or any other securities of the Company which may at any time be
issuable on the exercise hereof for any purpose, nor shall anything contained
herein be construed to confer upon the holder of this Warrant, as such, any of
the rights of a shareholder of the Company or any right to vote for the election
of directors or upon any matter submitted to shareholders at any meeting
thereof, or to receive notice of meetings, or to receive dividends or
subscription rights or otherwise until this Warrant shall have been exercised
and the Shares purchasable upon the exercise hereof shall have become
deliverable, as provided herein. Notwithstanding the foregoing, the Company will
transmit to the holder of this Warrant such information, documents and reports
as are generally distributed to the holders of any class or series of the
securities of the Company concurrently with the distribution thereof to the
shareholders.

      9.    Registration Rights. The Company hereby acknowledges that it has
granted Venture Lending & Leasing III, LLC registration rights pursuant to the
Registration Rights Agreement.

      10.   Additional Rights.

      10.1  Acquisition Transactions. The Company shall provide the holder of
this Warrant with at least twenty (20) days' written notice prior to closing
thereof of the terms and conditions of any of the following transactions (to the
extent the Company has notice thereof): (i) the sale, lease, exchange,
conveyance or other disposition of all or substantially all of the Company's
property or business, or (ii) its merger into or consolidation with any other
corporation (other than a wholly-owned subsidiary of the Company) in which more
than 50% of the voting power of the Company is disposed of, or any transaction
(including a merger or other reorganization) or series of related transactions,
in which more than 50% of the voting power of the Company is disposed of.

      10.2  Right to Convert Warrant into Stock: Net Issuance.

            (a)   Right to Convert. In addition to and without limiting the
rights of the holder under the terms of this Warrant, the holder shall have the
right to convert this Warrant or any portion thereof (the "Conversion Right")
into shares of Series Preferred (or Common Stock if the Series Preferred has
been automatically converted into Common Stock) as provided in this Section 10.2
at any time or from time to time during the term of this Warrant. Upon exercise
of the Conversion Right with respect to a particular number of shares subject to
this Warrant (the "Converted Warrant Shares"), the Company shall deliver to the
holder (without payment by the holder of any cash or other consideration) that
number of shares of fully paid and nonassessable Series Preferred (or Common
Stock if the Series Preferred has been converted into Common Stock) as is
determined according to the following formula:


                                     - 7 -
<PAGE>
<TABLE>
<S>                     <C>
        X =   B - A
            ---------
               Y

        Where:   X  =   the number of shares of Series Preferred (or Common
                        Stock if the Series Preferred has been converted to
                        Common Stock) that shall be issued to holder

                 Y  =   the fair market value of one share of Series Preferred
                        (or Common Stock if the Series Preferred has been
                        converted to Common Stock)

                 A  =   the aggregate Warrant Price of the specified number of
                        Converted Warrant Shares immediately prior to the
                        exercise of the Conversion Right (i.e., the number of
                        Converted Warrant Shares multiplied by the Warrant
                        Price)

                 B  =   the aggregate fair market value of the specified number
                        of Converted Warrant Shares (i.e., the number of
                        Converted Warrant Shares multiplied by the fair market
                        value of one Converted Warrant Share)
</TABLE>

      If the number of shares to be issued determined in accordance with the
foregoing formula is other than a whole number, the provisions of Section 6
above shall apply. For purposes of Section 10 of this Warrant, shares issued
pursuant to the Conversion Right shall be treated as if they were issued upon
the exercise of this Warrant.

            (b)   Method of Exercise. The Conversion Right may be exercised by
the holder by the surrender of this Warrant at the principal office of the
Company together with a written statement (which may be in the form of Exhibit
A-1 or Exhibit A-2 hereto) specifying that the holder thereby intends to
exercise the Conversion Right and indicating the number of shares subject to
this Warrant which are being surrendered (referred to in Section 10.2(a) hereof
as the Converted Warrant Shares) in exercise of the Conversion Right. Such
conversion shall be effective upon receipt by the Company of this Warrant
together with the aforesaid written statement, or on such later date as is
specified therein (the "Conversion Date"), and, at the election of the holder
hereof, may be made contingent upon the closing of the sale of the Company's
Common Stock to the public in a public offering pursuant to a Registration
Statement under the Act (a "Public Offering"). Certificates for the shares
issuable upon exercise of the Conversion Right and, if applicable, a new warrant
evidencing the balance of the shares remaining subject to this Warrant, shall be
issued as of the Conversion Date and shall be delivered to the holder within
thirty (30) days following the Conversion Date.

            (c)   Determination of Fair Market Value. For purposes of this
Section 10.2, "fair market value" of a share of Series Preferred (or Common
Stock if the Series Preferred has been converted into Common Stock) as of a
particular date (the "Determination Date") shall mean:

                  (i)   If the Conversion Right is exercised in connection with
and contingent upon a Public Offering, and if the Company's Registration
Statement relating to such Public Offering


                                     - 8 -
<PAGE>
("Registration Statement") has been declared effective by the Securities and
Exchange Commission, then the initial "Price to Public" specified in the final
prospectus with respect to such offering.

                  (ii)  If the Conversion Right is not exercised in connection
with and contingent upon a Public Offering, then as follows:

            (A)   If traded on a securities exchange, the fair market value of
the Common Stock shall be deemed to be the average of the closing prices of the
Common Stock on such exchange over the five trading days immediately prior to
the Determination Date, and the fair market value of the Series Preferred shall
be deemed to be such fair market value of the Common Stock multiplied by the
number of shares of Common Stock into which each share of Series Preferred is
then convertible;

            (B)   If traded on the Nasdaq Stock Market or other over-the-counter
system, the fair market value of the Common Stock shall be deemed to be the
average of the closing bid prices of the Common Stock over the five trading days
immediately prior to the Determination Date, and the fair market value of the
Series Preferred shall be deemed to be such fair market value of the Common
Stock multiplied by the number of shares of Common Stock into which each share
of Series Preferred is then convertible; and

            (C)   If there is no public market for the Common Stock, then fair
market value shall be determined in good faith by the Board of Directors of the
Company.

      10.3  Exercise Prior to Expiration. To the extent this Warrant is not
previously exercised as to all of the Shares subject hereto, and if the fair
market value of one share of the Series Preferred is greater than the Warrant
Price then in effect, this Warrant shall be deemed automatically exercised
pursuant to Section 10.2 above (even if not surrendered) immediately before its
expiration. For purposes of such automatic exercise, the fair market value of
one share of the Series Preferred upon such expiration shall be determined
pursuant to Section 10.2(c). To the extent this Warrant or any portion thereof
is deemed automatically exercised pursuant to this Section 10.3, the Company
agrees to promptly notify the holder hereof of the number of Shares, if any, the
holder hereof is to receive by reason of such automatic exercise.

      11.   Representations and Warranties. The Company represents and warrants
to the holder of this Warrant as follows:

            (a)   This Warrant has been duly authorized and executed by the
Company and is a valid and binding obligation of the Company enforceable in
accordance with its terms, subject to laws of general application relating to
bankruptcy, insolvency and the relief of debtors and the rules of law or
principles at equity governing specific performance, injunctive relief and other
equitable remedies, regardless of whether considered a proceeding in equity or
at law;

            (b)   The Shares have been duly authorized and reserved for issuance
by the Company and, when issued in accordance with the terms hereof, will be
validly issued, fully paid and non-assessable;


                                     - 9 -
<PAGE>
            (c)   The rights, preferences, privileges and restrictions granted
to or imposed upon the Series Preferred and the holders thereof are as set forth
in the Charter, and on the Date of Grant, each share of the Series Preferred
represented by this Warrant is convertible into one share of Common Stock;

            (d)   The shares of Common Stock issuable upon conversion of the
Shares have been duly authorized and reserved for issuance by the Company and,
when issued in accordance with the terms of the Charter will be validly issued,
fully paid and nonassessable;

            (e)   The execution and delivery of this Warrant are not, and the
issuance of the Shares upon exercise of this Warrant in accordance with the
terms hereof will not be, inconsistent with the Company's Charter or by-laws, do
not and will not contravene any law, governmental rule or regulation, judgment
or order applicable to the Company, and do not and will not conflict with or
contravene any provision of, or constitute a default under, any indenture,
mortgage, contract or other instrument of which the Company is a party or by
which it is bound or require the consent or approval of, the giving of notice
to, the registration or filing with or the taking of any action in respect of or
by, any Federal, state or local government authority or agency or other person,
except for the filing of notices pursuant to federal and state securities laws,
which filings will be effected by the time required thereby; and

            (f)   As of the date hereof, there are no actions, suits, audits,
investigations or proceedings pending or, to the knowledge of the Company,
threatened against the Company in any court or before any governmental
commission, board or authority which, if adversely determined, will have a
material adverse effect on the ability of the Company to perform its obligations
under this Warrant.

            (g)   The number of shares of Common Stock of the Company
outstanding on the date hereof, on a fully diluted basis (assuming the
conversion of all outstanding convertible securities and the exercise of all
outstanding options and warrants) does not exceed 29,000,000 shares.

      12.   Modification and Waiver. This Warrant and any provision hereof may
be changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

      13.   Notices. Any notice, request, communication or other document
required or permitted to be given or delivered to the holder hereof or the
Company shall be delivered, or shall be sent by certified or registered mail,
postage prepaid, to each such holder at its address as shown on the books of the
Company or to the Company at the address indicated therefor on the signature
page of this Warrant.

      14.   Binding Effect on Successors. This Warrant shall be binding upon any
corporation succeeding the Company by merger, consolidation or acquisition of
all or substantially all of the Company's assets, and all of the obligations of
the Company relating to the Series Preferred issuable upon the exercise or
conversion of this Warrant shall survive the exercise, conversion and


                                     - 10 -
<PAGE>
termination of this Warrant and all of the covenants and agreements of the
Company shall inure to the benefit of the successors and assigns of the holder
hereof.

      15.   Lost Warrants or Stock Certificates. The Company covenants to the
holder hereof that, upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant or any
stock certificate and, in the case of any such loss, theft or destruction, upon
receipt of an indemnity reasonably satisfactory to the Company, or in the case
of any such mutilation upon surrender and cancellation of such Warrant or stock
certificate, the Company will make and deliver a new Warrant or stock
certificate, of like tenor, in lieu of the lost, stolen, destroyed or mutilated
Warrant or stock certificate.

      16.   Descriptive Headings. The descriptive headings of the several
paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant. The language in this Warrant shall be
construed as to its fair meaning without regard to which party drafted this
Warrant.

      17.   Governing Law. This Warrant shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the State of Delaware.

      18.   Survival of Representations, Warranties and Agreements. All
representations and warranties of the Company and the holder hereof contained
herein shall survive the Date of Grant, the exercise or conversion of this
Warrant (or any part hereof) or the termination or expiration of rights
hereunder. All agreements of the Company and the holder hereof contained herein
shall survive indefinitely until, by their respective terms, they are no longer
operative.

      19.   Remedies. In case any one or more of the covenants and agreements
contained in this Warrant shall have been breached, the holders hereof (in the
case of a breach by the Company), or the Company (in the case of a breach by a
holder), may proceed to protect and enforce their or 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 breach and/or an action for specific performance
of any such covenant or agreement contained in this Warrant.

      20.   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
action as may be necessary or appropriate in order to protect the rights of the
holder of this Warrant against impairment.

      21.   Severability. The invalidity or unenforceability of any provision of
this Warrant in any jurisdiction shall not affect the validity or enforceability
of such provision in any other jurisdiction, or affect any other provision of
this Warrant, which shall remain in full force and effect.

      22.   Recovery of Litigation Costs. If any legal action or other
proceeding is brought for the enforcement of this Warrant, or because of an
alleged dispute, breach, default, or misrepresentation in connection with any of
the provisions of this Warrant, the successful or


                                     - 11 -
<PAGE>
prevailing party or parties shall be entitled to recover reasonable attorneys'
fees and other costs reasonably incurred in that action or proceeding, in
addition to any other relief to which it or they may be entitled.

      23.   Entire Agreement; Modification. This Warrant constitutes the entire
agreement between the parties pertaining to the subject matter contained in it
and supersedes all prior and contemporaneous agreements, representations, and
undertakings of the parties, whether oral or written, with respect to such
subject matter.


                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]


                                     - 12 -
<PAGE>
      The Company and the holder of this Warrant have caused this Warrant to be
duly executed and delivered as of the Date of Grant specified above.

                                           ACUSPHERE, INC.


                                           By: /s/ Sherri C. Oberg
                                               --------------------------------

                                           Name: Sherri C. Oberg

                                           Title: President

                                           Address: 38 Sidney Street

                                                    Cambridge, MA 02139



                                           VENTURE LENDING & LEASING III, LLC


                                           By: /s/ Salvador O. Gutierrez
                                               --------------------------------

                                           Name: Salvador O. Gutierrez
                                               --------------------------------

                                           Title: President
                                               --------------------------------


                                     - 13 -
<PAGE>
                                   EXHIBIT A-1


                               NOTICE OF EXERCISE


To:   ACUSPHERE, INC. (the "Company")


      1.    The undersigned hereby:

            [ ]   elects to purchase________ shares of [Series F Preferred
                  Stock] [Common Stock] of the Company pursuant to the terms
                  of the attached Warrant, and tenders herewith payment of
                  the purchase price of such shares in full, or

            [ ]   elects to exercise its net issuance rights pursuant to
                  Section 10.2 of the attached Warrant with respect
                  to________Shares of [Series F Preferred Stock] [Common Stock].

      2.    Please issue a certificate or certificates representing ________
shares in the name of the undersigned or in such other name or names as are
specified below:


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


                 ---------------------------------------------

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

      3.    The undersigned represents that the aforesaid shares are being
acquired for the account of the undersigned for investment and not with a view
to, or for resale in connection with, the distribution thereof and that the
undersigned has no present intention of distributing or reselling such shares,
all except as in compliance with applicable securities laws.


                                   ---------------------------------------------
                                   (Signature)


------------------
    (Date)
<PAGE>
                                   EXHIBIT A-2

                               NOTICE OF EXERCISE

To:   ACUSPHERE, INC. (the "Company")

      1.    Contingent upon and effective immediately prior to the closing (the
"Closing") of the Company's public offering contemplated by the Registration
Statement on Form S___, filed________, 200_, the undersigned hereby:

      [ ]   elects to purchase________shares of [Series F Preferred Stock]
[Common Stock] of the Company (or such lesser number of shares as may be sold on
behalf of the undersigned at the Closing) pursuant to the terms of the attached
Warrant, or

      [ ]   elects to exercise its net issuance rights pursuant to Section
10.2 of the attached Warrant with respect to________Shares of [Series F
Preferred Stock] [Common Stock].

      2.    Please deliver to the custodian for the selling shareholders a stock
certificate representing such________shares.

      3.    The undersigned has instructed the custodian for the selling
shareholders to deliver to the Company $________or, if less, the net proceeds
due the undersigned from the sale of shares in the aforesaid public offering. If
such net proceeds are less than the purchase price for such shares, the
undersigned agrees to deliver the difference to the Company prior to the
Closing.



                                   ---------------------------------------------
                                   (Signature)


------------------
    (Date)
<PAGE>
                                    EXHIBIT B

                                     CHARTER