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                                                                     Exhibit 4.1

                             FORM OF CLASS A WARRANT

NEITHER THIS WARRANT NOR THE SECURITIES FOR WHICH THIS WARRANT IS EXERCISABLE
HAVE BEEN REGISTERED WITH THE SECURITIES AND EXCHANGE COMMISSION OR THE
SECURITIES COMMISSION OF ANY STATE IN RELIANCE UPON EXEMPTIONS FROM REGISTRATION
UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT") AND
APPLICABLE STATE LAWS, AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD EXCEPT
PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OR
PURSUANT TO AN AVAILABLE EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE
REGISTRATION REQUIREMENTS OF THE SECURITIES ACT AND IN ACCORDANCE WITH
APPLICABLE STATE SECURITIES LAWS.

IN ADDITION, THE WARRANTS REPRESENTED HEREBY ARE SUBJECT TO CERTAIN RESTRICTIONS
ON TRANSFER AND CONVERSION SET FORTH IN THE SECURITIES PURCHASE AGREEMENT DATED
AS OF SEPTEMBER 22, 1999 AMONG THE COMPANY AND THE PURCHASERS PARTY THERETO, A
COPY OF WHICH IS ON FILE IN THE OFFICE OF THE COMPANY.

September 22, 1999
1,044,686 shares                                        Warrant No. 2002-Reset A

                           BOSTON LIFE SCIENCES, INC.
                             STOCK PURCHASE WARRANT

Registered Owner: Brown Simpson Partners I, Ltd.

     This certifies that, for value received, Boston Life Sciences, Inc., a
Delaware corporation, the ("Company") grants the following rights to the
Registered Owner, or assigns, of this Warrant:

     1.   Issue. Upon tender (as defined in Section 5 hereof) to the Company,
the Company, within three (3) Business Days of the date thereof, shall issue to
the Registered Owner, or assigns, up to the number of shares specified in
Section 2 hereof of fully paid and nonassessable shares of Common Stock that the
Registered Owner, or assigns, is otherwise entitled to purchase.

     2.   Number of Shares. The total number of shares of Common Stock that the
Registered Owner, or assigns, of this Warrant is entitled to receive upon
exercise of this Warrant (the "Warrant Shares") is 1,044,686 shares, subject to
adjustment from time to time as to the number and kind of securities for which
this Warrant is exercisable, all as set forth in Section 6 hereof. The Company
shall at all times reserve and hold available out of its authorized and unissued
shares of Common Stock or other securities, as the case may be, sufficient
shares of Common Stock to satisfy all conversion and purchase rights represented
by outstanding convertible securities, options and warrants, including this
Warrant. The Company covenants and agrees that all shares of Common Stock or
other securities, as the case may be, that may be issued upon the exercise of
this Warrant shall, upon issuance, be duly and validly issued, fully paid and
nonassessable, free from all taxes, liens and charges with respect to the
purchase and the issuance of the shares, and shall not have any legend or
restrictions on resale, except as required by Section 3.2(b) of the Purchase
Agreement.

     3.   Exercise Price.

          a. The initial per share exercise price of this Warrant, representing
     the price per share at which the shares of stock issuable upon exercise of
     this Warrant may be purchased, is $2.15, subject to adjustment from time to
     time pursuant to the provisions of Section 6 hereof (the "Exercise Price").

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                b. At the option of the Registered Owner, the Exercise Price
         will be reduced to an amount which is equal to the sum of (x) the
         product of (i) 4.5% and (ii) the average of the Per Share Market Value
         for the seven (7) Trading Days immediately following the occurrence of
         either of the following events (each, a "Reset Event") and (y) the
         average of the Per Share Market Value for the seven (7) Trading Days
         immediately following the occurrence of such Reset Event:

                   (i)    The Company issues shares of its Common Stock or
                options, warrants or other securities convertible or
                exchangeable therefor (other than Excluded Securities), at a
                price per share of Common Stock below the Exercise Price then
                in effect; or

                   (ii)   If all or any portion of the principal amount of the
                Debenture (as defined in the Purchase Agreement is outstanding
                on the first anniversary of the Closing Date.

                   (iii)  If all or any portion of the principal amount of the
                Debenture (as defined in the Purchase Agreement) is outstanding
                on the second anniversary of the Closing Date.

In the case of a Reset Event under (i) above, (a) the Company shall notify the
Majority Holders in writing within five (5) Business Days of the date of such
event (the "Company Reset Notice") in accordance with Section 6(e), and (b) the
Reset Option will be deemed to be exercised, and the Exercise Price shall be
automatically reduced as of the date of such Reset Event unless, within five (5)
Business Days after the receipt by the Holder of the Company Reset Notice, the
Registered Holder provides the Company with written notification of its decision
not to exercise the Reset Option. In the case of a Reset Event under (ii) or
(iii) above, the Conversion Price shall be automatically reduced upon the
occurrence of such Reset Event. Notwithstanding the foregoing, in the case of a
Reset Event under (ii) above, the Exercise Price shall not be reduced below
$3.14, and in the case of a Reset Event under (iii) above, the Exercise Price
shall not be reduced below $1.57.

         4. Exercise Period. This Warrant may be exercised from the Closing Date
(as defined in the Purchase Agreement) up to and including September 22, 2004 (5
years less 1 day) (the "Exercise Period"). If not exercised during this period,
this Warrant and all rights granted under this Warrant shall expire and lapse.

         5. Tender; Issuance of Certificates.

                a. Subject to Section 16 hereof, this Warrant may be exercised,
         in whole or in part, by (i) actual delivery of (a) the Exercise Price
         in cash, (b) a duly executed Warrant Exercise Form, a copy of which is
         attached to this Warrant as Exhibit A, properly executed by the
         Registered Owner, or assigns, of this Warrant, and (c) by surrender of
         this Warrant, or (ii) if the resale of the Warrant Shares by the
         Registered Owner is not then registered pursuant to an effective
         registration statement under the Securities Act, delivery to the
         Company of a written notice of an election to effect a "Cashless
         Exercise" in accordance with Section 5(b), below, for the Warrant
         Shares specified in the Warrant Exercise Form. The Warrant Shares so
         purchased shall be deemed to be issued to the Registered Owner as of
         the close of business on the date on which this Warrant shall have been
         surrendered, the completed Warrant Exercise Form shall have been
         delivered and payment shall have been made for such shares as set forth
         above. The payment and Warrant Exercise Form must be delivered to the
         registered office of the Company either in person or as set for in
         Section 13 hereof.

                b. Commencing one hundred (100) days from the Filing Date (as
         defined in the Registration Rights Agreement), if, and only if, at the
         time of exercise of this Warrant, the Warrant Shares are not saleable
         pursuant to an effective registration statement, other than as allowed
         in Section 3(r) of the Registration Rights Agreement, then in addition
         to the exercise of all or a part of this Warrant by payment of the
         Exercise Price in cash as provided above, and in lieu of such payment,
         the Registered Owner shall have the right to effect a cashless exercise
         (a "Cashless Exercise"). In the event of a Cashless Exercise the
         Registered Owner may exercise this

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         Warrant in whole or in part by surrendering this Warrant in exchange
         for the number of shares of Common Stock equal to the product of (x)
         the number of shares as to which this Warrant is being exercised
         multiplied by (y) a fraction, the numerator of which is the Per Share
         Market Value of the Common Stock less the Exercise Price then in effect
         and the denominator of which is the Per Share Market Value (in each
         case adjusted for fractional shares as herein provided).

                c. In lieu of physical delivery of the Warrant Shares, provided
         the Company's transfer agent is participating in the Depositary Trust
         Company ("DTC") Fast Automated Securities Transfer ("FAST") program,
         upon consent of the Registered Owner and in compliance with the
         provisions hereof, the Company may, at its option, use its best efforts
         to cause its transfer agent to electronically transmit the Warrant
         Shares to the Registered Owner by crediting the account of the
         Registered Owner's Prime Broker with DTC through its Deposit Withdrawal
         Agent Commission system. The time period for delivery described herein
         shall apply to the electronic transmittals described in subparagraph
         (d) below.

                d. Certificates for the Warrant Shares so purchased,
         representing the aggregate number of shares specified in the Warrant
         Exercise Form, and any cash payments due under Section 15 hereof shall
         be delivered to the Registered Owner within a reasonable time, not
         exceeding three (3) Business Days, after this Warrant shall have been
         so exercised. The certificates so delivered shall be in such
         denominations as may be requested by the Registered Owner and shall be
         registered in the name of the Registered Owner or such other name as
         shall be designated by such Registered Owner. If this Warrant shall
         have been exercised only in part, then, unless this Warrant has
         expired, the Company shall, at its expense, at the time of delivery of
         such certificates, deliver to the Registered Owner a new Warrant
         representing the number of shares with respect to which this Warrant
         shall not then have been exercised.

         6.       Adjustment of Exercise Price.

                a. Common Stock Dividends; Common Stock Splits; Reverse Common
         Stock Splits. If the Company, at any time while this Warrant is
         outstanding, (a) shall pay a stock dividend on its Common Stock, (b)
         subdivide outstanding shares of Common Stock into a larger number of
         shares, (c) combine outstanding shares of Common Stock into a smaller
         number of shares or (d) issue by reclassification of shares of Common
         Stock any shares of capital stock of the Company, then the Exercise
         Price shall be multiplied by a fraction the numerator of which shall be
         the number of shares of Common Stock (excluding treasury shares, if
         any) outstanding before such event and the denominator of which shall
         be the number of shares of Common Stock outstanding after such event.
         Any adjustment made pursuant to this paragraph (6)(a) shall become
         effective immediately after the record date for the determination of
         shareholders entitled to receive such dividend or distribution and
         shall become effective immediately after the effective date in the case
         of a subdivision, combination or re-classification.

                b. Rights; Options; Warrants or Other Securities. If the
         Company, at any time while this Warrant is outstanding, shall issue
         rights, options, warrants or other securities to all of the holders of
         Common Stock entitling them to subscribe for or purchase, convert to,
         exchange for or otherwise acquire shares of Common Stock for no
         consideration or at a price per share less than the Threshold Price,
         the Exercise Price shall be multiplied by a fraction, the denominator
         of which shall be the number of shares of Common Stock (excluding
         treasury shares, if any) outstanding on the date of issuance of such
         rights, options, warrants or other securities plus the number of
         additional shares of Common Stock offered for subscription, purchase,
         conversion, exchange or acquisition and the numerator of which shall be
         the number of shares of Common Stock (excluding treasury shares, if
         any) outstanding on the date of issuance of such rights, options,
         warrants or other securities plus the number of shares which the
         aggregate offering price of the total number of shares so offered would
         purchase at the Exercise Price. Such adjustment

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         shall be made whenever such rights, options, warrants or other
         securities are issued, and shall become effective immediately after the
         issue date of such rights, options, warrants or other securities.
         However, upon the expiration of any rights, warrants, options or other
         securities, the issuance of which resulted in an adjustment in the
         Exercise Price pursuant to this Section 6(b), if any such rights,
         warrants, options or other securities shall expire and all or any
         portion thereof shall not have been exercised, the Exercise Price shall
         immediately upon such expiration be re-computed and effective
         immediately upon such expiration be increased to the price which it
         would have been (but reflecting any other adjustments in the Exercise
         Price made pursuant to the provisions of Section 6(h) after the
         issuance of such rights, warrants, options or other securities) had the
         adjustment of the Exercise Price made upon the issuance of such rights,
         warrants, options or other securities been made on the basis of the
         issuance of only that number of shares of Common Stock (if any)
         actually purchased upon the exercise of such rights, warrants, options
         or other securities actually exercised.

                c. Subscription Rights. If the Company, at any time while this
         Warrant is outstanding, shall distribute to all of the holders of
         Common Stock evidence of its indebtedness or assets or rights, options,
         warrants or other security entitling them to subscribe for or purchase,
         convert to, exchange for or otherwise acquire any security (excluding
         those referred to in paragraphs 6(a) and (b) above), then in each such
         case the Exercise Price at which the Warrant shall thereafter be
         exercisable shall be determined by multiplying the Exercise Price in
         effect immediately prior to the date of such distribution by a
         fraction, the denominator of which shall be the Per Share Market Value
         of Common Stock determined as of the record date mentioned above, and
         the numerator of which shall be such Per Share Market Value of the
         Common Stock on such date less the then fair market value at such
         record date of the portion of such assets or evidence of indebtedness
         so distributed applicable to one outstanding share of Common Stock as
         determined by the Board of Directors in good faith; provided, however,
         that in the event of a distribution exceeding ten percent (10%) of the
         net assets of the Company, such fair market value shall be determined
         by an Appraiser selected in good faith by the Registered Owner of the
         Warrant; and provided, further, that the Company, after receipt of the
         determination by such Appraiser shall have the right to select in good
         faith an additional Appraiser meeting the same qualifications in which
         case the fair market value shall be equal to the average of the
         determinations by each such Appraiser. Such adjustment shall be made
         whenever any such distribution is made and shall become effective
         immediately thereafter.

                d. Rounding. All calculations under this Section 6 shall be made
         to the nearest cent or the nearest l/l00th of a share, as the case may
         be.

                e. Notice of Adjustment. Whenever the Exercise Price is adjusted
         pursuant to paragraphs 3(b) or 6(a), (b), (c) or (h), the Company shall
         promptly deliver to the Registered Owner a notice setting forth the
         Exercise Price after such adjustment and setting forth a brief
         statement of the facts requiring such adjustment.

                f. Reclassification, Etc. In the event of any reclassification
         of the Common Stock or any compulsory share exchange pursuant to which
         the Common Stock is converted into other securities, cash or property,
         the Registered Owner shall have the right thereafter to convert the
         shares of Common Stock underlying this Warrant into the securities,
         cash or property receivable upon or deemed to be held by holders of
         Common Stock following such reclassification or compulsory share
         exchange, and the Registered Owner shall be entitled upon such event to
         receive such amount of securities, cash or property received by a
         holder of Common Stock in an amount equal to the difference of (i) the
         amount such holder would have received had he held the number of shares
         of Common Stock for which this Warrant could have been exercised
         immediately prior to such reclassification or compulsory share exchange
         minus (ii) the Exercise Price in effect immediately prior to such date,
         multiplied by the number of shares of Common Stock for which this
         Warrant could have been exercised immediately prior to such
         reclassification or compulsory share exchange.

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         g.    Notice of Certain Events. If:

               (i)   the Company shall declare a dividend (or any other
         distribution) on its Common Stock; or

               (ii)  the Company shall declare a special nonrecurring cash
         dividend on or a redemption of its Common Stock; or

               (iii) the Company shall authorize the granting to the holders of
         the Common Stock rights or warrants to subscribe for or purchase any
         shares of capital stock of any class or of any rights; or

               (iv)  the approval of any shareholders of the Company shall be
         required in connection with any reclassification of the Common Stock of
         the Company, any consolidation or merger to which the Company is a
         party, any sale or transfer of all or substantially all of the assets
         of the Company, or any compulsory share exchange whereby the Common
         Stock is converted into other securities, cash or property; or

               (v)   the Company shall authorize the voluntary or involuntary
         dissolution, liquidation or winding up of the affairs of the Company;

then the Company shall cause to be filed at each office or agency maintained for
the purpose of exercise of this Warrant, and shall cause to be delivered to the
Registered Owner, at least 10 Business Days prior to the applicable record or
effective date hereinafter specified, a notice (provided such notice shall not
include any material non-public information) stating (x) the date on which a
record is to be taken for the purpose of such dividend, distribution,
redemption, rights or warrants, or if a record is not to be taken, the date as
of which the holders of Common Stock of record to be entitled to such dividend,
distributions, redemption, rights or warrants are to be determined or (y) the
date on which such reclassification, consolidation, merger, sale, transfer or
share exchange is expected to become effective or close, and the date as of
which it is expected that holders of Common Stock of record shall be entitled to
exchange their shares of Common Stock for securities, cash or other property
deliverable upon such reclassification, consolidation, merger, sale, transfer or
share exchange; provided, however, that the failure to mail such notice or any
defect therein or in the mailing thereof shall not affect the validity of the
corporate action required to be specified in such notice.

         h.    Adjustment to Exercise Price. If the Company, at any time while
    this Warrant is outstanding, takes any of the actions described in this
    Section 6(h), then, in order to prevent dilution of the rights granted under
    this Warrant, the Exercise Price will be subject to adjustment from time to
    time as provided in this Section 6(h); provided, however, that no
    adjustment to the Conversion Price shall be made pursuant to this Section
    6(h) in respect of any action by the Company with respect to which the
    Conversion Price is adjusted pursuant to Section 3(b) above.

               (i)   Adjustment of Exercise Price upon Issuance of Common Stock.
         If at any time while this Warrant is outstanding the Company issues or
         sells, or is deemed to have issued or sold, any shares of Common Stock
         (other than any Excluded Securities) for a consideration per share less
         than the lower of the Per Share Market Value on the Trading Day
         immediately prior to the date of issuance or sale and (y) the Exercise
         Price (such lesser price, the "Threshold Price"), the Exercise Price
         then in effect shall be reduced to an amount equal to the consideration
         per share of Common Stock of such issuance or sale. For the purpose of
         determining the adjusted Exercise Price under this Section 6(h)(i), the
         following shall be applicable:

               (A) Issuance of Options. If at any time while this Warrant is
         outstanding the Company in any manner grants, issues or sells any
         rights, options, warrants, options to subscribe for or to purchase
         Common Stock or any stock or other securities convertible

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                  into or exchangeable for Common Stock (other than any Excluded
                  Securities) (such rights, option or warrants being herein
                  called "Options" and such convertible or exchangeable stock or
                  securities being herein called "Convertible Securities") and
                  the price per share for which Common Stock is issuable upon
                  the exercise of such Options or upon conversion or exchange of
                  such Convertible Securities is less than the Threshold Price,
                  then the Exercise Price shall be adjusted to equal the price
                  per share for which Common Stock is issuable upon the exercise
                  of such Options or upon the conversion or exchange of such
                  Convertible Securities. No adjustment of the Exercise Price
                  shall be made upon the actual issuance of such Common Stock or
                  of such Convertible Securities upon the exercise of such
                  Options or upon the actual issuance of such Common Stock upon
                  conversion or exchange of such Convertible Securities.

                      (B) Issuance of Convertible Securities. If at any time
                  while this Warrant is outstanding the Company in any manner
                  issues or sells any Convertible Securities and the price per
                  share for which Common Stock is issuable upon such conversion
                  or exchange (other than any Excluded Securities) is less than
                  the Threshold Price, then the Exercise Price shall be adjusted
                  to equal the price per share for which Common Stock is
                  issuable upon the conversion or exchange of such Convertible
                  Securities.

                      (C) Change in Option Price or Rate of Conversion. If there
                  is a change at any time in (i) the purchase price provided for
                  in any Options, (ii) the additional consideration, if any,
                  payable upon the issuance, conversion or exchange of any
                  Convertible Securities or (iii) the rate at which any
                  Convertible Securities are convertible into or exchangeable
                  for Common Stock, then the Exercise Price in effect at the
                  time of such change shall be readjusted to the Exercise Price
                  which would have been in effect at such time had such Options
                  or Convertible Securities still outstanding provided for such
                  changed purchase price, additional consideration or changed
                  conversion rate, as the case may be, at the time initially
                  granted, issued or sold; provided that no adjustment shall be
                  made if such adjustment would result in an increase of the
                  Exercise Price then in effect.

                      (D) Effect on Exercise Price of Certain Events. For
                  purposes of determining the adjusted Exercise Price under this
                  Section 6(h)(i), the following shall be applicable:

                          (I) Calculation of Consideration Received. If any
                      Common Stock, Options or Convertible Securities are issued
                      or sold or deemed to have been issued or sold for cash,
                      the consideration received therefor will be deemed to be
                      the net amount received by the Company therefor. In case
                      any Common Stock, Options or Convertible Securities are
                      issued or sold for a consideration other than cash, the
                      amount of the consideration other than cash received by
                      the Company will be the fair value of such consideration,
                      except where such consideration consists of securities, in
                      which case the amount of consideration received by the
                      Company in respect of such securities will be based on the
                      Average Price of such securities on the Trading Day
                      immediately preceding the date of receipt thereof. In case
                      any Common Stock, Options or Convertible Securities are
                      issued to the owners of the non-surviving entity in
                      connection with any merger in which the Company is the
                      surviving entity, the amount of consideration therefor
                      will be deemed to be the fair value of such portion of the
                      net assets and business of the non-surviving entity as is
                      attributable to such Common Stock, Options or Convertible
                      Securities, as the case may be. The fair value of any
                      consideration other than cash or securities will be
                      determined jointly by the Company and the registered
                      owners of at least 60% of the Underlying Shares and
                      Warrants then outstanding. If such parties are unable to
                      reach agreement within ten (10) days after the occurrence
                      of an event requiring valuation (the "Valuation Event"),
                      the fair value of such consideration will be determined
                      within forty-eight (48) hours of the tenth (10th) day
                      following the Valuation Event by an Appraiser selected in

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                    good faith by the Company and agreed upon in good faith by
                    the holders of at least 60% of the Underlying Shares and the
                    Warrants then outstanding. The determination of such
                    Appraiser shall be binding upon all parties absent manifest
                    error.

                         (II)  Integrated Transactions. In case any Option is
                    issued in connection with the issue or sale of other
                    securities of the Company, together comprising one
                    integrated transaction in which no specific consideration is
                    allocated to such Options by the parties thereto, the
                    Options will be deemed to have been issued for an aggregate
                    consideration of $ .01.

                         (III) Treasury Shares. The number of shares of Common
                    Stock outstanding at any given time does not include shares
                    owned or held by or for the account of the Company, and the
                    disposition of any shares so owned or held will be
                    considered an issue or sale of Common Stock.

                         (IV)  Record Date. If the Company takes a record of the
                    holders of Common Stock for the purpose of entitling them
                    (1) to receive a dividend or other distribution payable in
                    Common Stock, Options or in Convertible Securities or (2) to
                    subscribe for or purchase Common Stock, Options or
                    Convertible Securities, then such record date will be deemed
                    to be the date of the issue or sale of the shares of Common
                    Stock deemed to have been issued or sold upon the
                    declaration of such dividend or the making of such other
                    distribution or the date of the granting of such right of
                    subscription or purchase, as the case may be.

                         (V)   Certain Events. If any event that would adversely
                    affect the rights of the Registered Owner of this Warrant
                    occurs but is not expressly provided for by Section 6 hereof
                    (including, without limitation, the granting of stock
                    appreciation rights, phantom stock rights or other rights
                    with equity features), then the Company's Board of Directors
                    will make an appropriate adjustment in the Exercise Price so
                    as to protect the rights of the Registered Owner of this
                    Warrant; provided, however, that no such adjustment will
                    increase the Exercise Price as otherwise determined pursuant
                    to this Section 6(h).

               Notwithstanding the foregoing, in no event shall any provision in
         this Section 6 cause the Exercise Price to be greater than the Exercise
         Price on the date of issuance of this Warrant.

               i.   Adjustment of Number of Shares. Upon each adjustment of the
         Exercise Price as a result of the calculations made in this Section 6,
         the number of Warrant Shares shall be multiplied by a fraction, the
         numerator of which shall be the number of shares of Common Stock
         (excluding treasury shares, if any) outstanding after such event and
         the denominator of which shall be the number of shares of Common Stock
         (excluding treasury shares, if any) outstanding before such event.

         7.    Restriction on Conversion by Either the Registered Owner or the
Company. Notwithstanding anything herein to the contrary, in no event shall any
Registered Owner or the Company have the right or be required to exercise this
Warrant if as a result of such conversion the aggregate number of shares of
Common Stock beneficially owned by such Registered Owner and its Affiliates
would exceed 9.99% of the outstanding shares of the Common Stock following such
exercise. For purposes of this Section 7, beneficial ownership shall be
calculated in accordance with Section 13(d) of the Securities Exchange Act of
1934, as amended. The provisions of this Section 7 may be waived by a Registered
Owner as to itself (and solely as to itself) upon not less than 65 days prior
written notice to the Company, and the provisions of this Section 7 shall
continue to apply until such 65th day (or later, if stated in the notice of
waiver).

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         8. Officer's Certificate. Whenever the number of shares purchasable
upon exercise shall be adjusted as required by the provisions of Section 3(b) or
Section 6, the Company shall forthwith file in the custody of its Secretary or
an Assistant Secretary at its principal office and with its stock transfer
agent, if any, an officer's certificate showing the adjusted Exercise Price,
number of shares or other securities determined as herein provided, setting
forth in reasonable detail the facts requiring such adjustment and the manner of
computing such adjustment. Each such officer's certificate shall be signed by
the chairman, president or chief financial officer of the Company and by the
secretary or any assistant secretary of the Company. Each such officer's
certificate shall be made available at all reasonable times for inspection by
any Registered Owner of the Warrants and the Company shall, forthwith after each
such adjustment, deliver a copy of such certificate to the each of the
Registered Owners.

         9. Definitions. Capitalized terms used herein and not otherwise defined
herein shall have the meanings given to such terms in the Purchase Agreement. As
used in this Warrant, the following terms have the following meanings:

         "Affiliate" means, with respect to any Person, any other Person that
directly or indirectly controls or is controlled by or under common control with
such Person. For the purposes of this definition, "control," when used with
respect to any Person, means the possession, direct or indirect, of the power to
direct or cause the direction of the management and policies of such Person,
whether through the ownership of voting securities, by contract or otherwise;
and the terms of "affiliated," "controlling" and "controlled" have meanings
correlative to the foregoing.

         "Appraiser" shall mean a nationally recognized or major regional
investment banking firm or firm of independent certified public accountants of
recognized standing.

         "Approved Stock Plan" shall mean any contract, plan or agreement which
has been approved by the Board of Directors of the Company, pursuant to which
the Company's securities may be issued to any employee, officer, director or
consultant or other person with a business relationship with the Company.

         "Business Day" means any day except Saturday, Sunday and any day which
shall be a legal holiday or a day on which banking institutions in the state of
New York generally are authorized or required by law or other government actions
to close.

         "Change of Control" has the meaning set forth in Section 4.1 of the
Purchase Agreement.

         "Closing" has the meaning set forth in Section 1.2 of the Purchase
Agreement.

         "Common Stock" means the shares of the Company's Common Stock, par
value $0.01 per share.

         "Company" means Boston Life Sciences, Inc., a Delaware corporation.

         "Convertible Securities" has the meaning assigned to it in Section
6(h)(i)(A) hereof.

         "Excluded Securities" means (i) the Debentures, the Warrants and the
Underlying Shares, (ii) shares of Common Stock of the Company, or any options,
warrants or other securities convertible or exchangeable therefor, that are
issued or deemed to have been issued from time to time in connection with an
Approved Stock Plan, (iii) shares of Common Stock of the Company issuable upon
the exercise of any options, warrants or other securities convertible or
exercisable therefor outstanding on the date hereof and listed in Schedule
2.1(c) of the Purchase Agreement, (iv) shares of Common Stock of the Company, or
any options, warrants or other securities convertible or exchangeable therefor,
that are issued or deemed to have been issued as consideration for an
acquisition by the Company of a division, assets or business (or stock
constituting any portion thereof) from another Person, (v) warrants for up to
200,000 shares of Common Stock per year, not exceeding 600,000 shares in the
aggregate, provided that the conversion price of any such warrants is above the
Per Share Market Value on the date the warrant is issued, (and the Common Stock
issued upon exercise thereof) issued after the date hereof by the Company for
purposes

                                       8

<PAGE>

other than raising capital, (vi) either Common Stock or preferred stock
convertible into Common Stock, provided that such stock shall be issued at a
price per share of not less than $4.50 in return for proceeds not to exceed the
greater of (1) $2,000,000 or (2) the amount expended by the Company in the
redemption of the Company's Series C floating preferred stock; provided, further
however, that (a) such stock shall not be issued prior to December 1, 1999 and
resales of such Common Stock (or Common Stock issuable upon conversion of such
preferred stock) shall not be registered under the Securities Act prior to April
1, 2000, (b) the purchasers of such Common Stock shall be prohibited from
executing short sales of Common Stock under substantially the same terms and
conditions as the Purchasers are hereunder, and (c) Brown Simpson shall have a
right of first offer with respect to such sale of stock at a price that is not
less than the price at which such stock is eventually sold; and provided further
that if preferred stock is issued, the conversion terms of such preferred stock
shall be no more favorable to investors than the conversion terms in the
Debenture and (vii) warrants described on Schedule 2.1(m) to the Securities
Purchase Agreement.

         "Exercise Period" has the meaning assigned to it the Section 4 hereof.

         "Exercise Price" has the meaning assigned to it in Section 3 hereof.

         "Options" has the meaning assigned to it in Section 6(h)(i)(A) hereof.

         "Per Share Market Value" means on any particular date (i) the closing
bid price per share of the Common Stock on such date on the Nasdaq Smallcap
Market or other registered national stock exchange on which the Common Stock is
then listed or if there is no such price on such date, then the closing bid
price on such exchange or quotation system on the date nearest preceding such
date, or (ii) if the Common Stock is not listed then on the Nasdaq Smallcap
Market or any registered national stock exchange, the closing bid price for a
share of Common Stock in the over-the-counter market, as reported by the
National Quotation Bureau Incorporated (or similar organization or agency
succeeding to its functions of reporting prices) at the close of business on
such date, or (iii) if the Common Stock is not then publicly traded the fair
market value of a share of Common Stock as determined by an Appraiser selected
in good faith by the holders of at least 60% of the underlying shares issuable
upon exercise of the warrants; provided, however, that the Company, after
receipt of the determination by such Appraiser, shall have the right to select,
in good faith, an additional Appraiser, in which case the fair market value
shall be equal to the average of the determinations by each such Appraiser; and
provided, further that all determinations of the Per Share Market Value shall be
appropriately adjusted for any stock dividends, stock splits or other similar
transactions during such period.

         "Purchase Agreement" means that certain Securities Purchase Agreement,
dated September 22, 1999, among the Company and the Purchasers.

         "Purchaser" has the meaning set forth in the Purchase Agreement.

         "Registered Owner" means the person identified on the face of this
Warrant as the registered owner hereof or such other person as shown on the
records of the Company as being the registered owner of this Warrant or their
assigns.

         "Registration Rights Agreement" means that certain Registration Rights
Agreement, dated September 22, 1999, among the Company and the Purchasers.

         "Reset Event" has the meaning assigned to it in Section 3(b) hereof.

         "Threshold Price" has the meaning set forth in Section 6(h)(i) hereof.

         "Trading Day(s)" means any day on which the primary market on which
shares of Common Stock are listed is open for trading.

                                       9

<PAGE>

         "Underlying Shares" has the meaning assigned to it in Section 2.1(d) of
the Purchase Agreement.

         "Warrant(s)" means the warrants issuable at the Closing.

         10. NASDAQ Limitation. The Company covenants that if any shares of
Common Stock required to be reserved for purposes of conversion of Debentures
hereunder require registration with or approval of any governmental authority
under any Federal or state law, or any national securities exchange, before such
shares may be issued upon conversion, the Company will use its best efforts to
cause such shares to be duly registered or approved, as the case may be.

         If on any date (the "Determination Date") (a) the Common Stock is
listed for trading on Nasdaq or the Nasdaq National Market, (b) the Exercise
Price then in effect is such that the sum of (x) the aggregate number of shares
of Common Stock that would then be issuable upon conversion in full of the then
outstanding principal amount of the Debentures and exercise in full of the
Warrants as if all such Debentures were converted and such warrants were
exercised on such Determination Date (without regard to any limitations on
conversions) and as payment of interest thereon, and (y) the aggregate number of
shares of Common Stock issued to the Purchasers upon any prior conversion of the
Debentures or exercise of the Warrants, would equal or exceed 20% of the number
of shares of the Common Stock outstanding on the Closing Date (such number of
shares as would not equal or exceed such 20% limit, the "Issuable Maximum"), and
(c) the Company shall not have previously obtained the vote of the shareholders
of the Company (the "Shareholder Approval"), if any, as may be required by the
applicable rules and regulations of Nasdaq (or any successor entity) to approve
the issuance of shares of Common Stock in excess of the Issuable Maximum in a
private placement whereby shares of Common Stock are deemed to have been issued
at a price that is less than the greater of book value or fair market value of
the Common Stock, then with respect to the number of Warrant Shares the issuance
of which would result in an issuance of shares of Common Stock in excess of the
Warrant Holder's (a "Holder") pro rata allocation (as described below) of the
Issuable Maximum (the "Excess Warrant Shares"), the Company may elect to prepay
cash to the Holders in an amount equal to the product of (A) the difference
between the Per Share Market Value and the Exercise Price, multiplied by (B) the
pro rata allocation of such Holder's Excess Warrant Shares (the "Non-Exercise
Payment Amount"). Any such election by the Company must be made in writing to
the Holders within five (5) Trading Days after the Determination Date and the
payment of such Non-Exercise Prepayment Amount must be made in full to the
Holders within ten (10) Business Days after the date such notice is delivered.
If the Company does not deliver timely a notice of its election to prepay under
this Section or shall, if it shall have delivered such a notice, fail to pay the
Non-Exercise Payment Amount hereunder within ten (10) Business Days thereafter,
then the holders of at least 60% of the Underlying Shares exercisable upon
exercise of the Warrants shall have the option by written notice to the Company,
to declare any such notice given by the Company, if given, to be null and void
and require the Company to either: (i) use its best efforts to obtain the
Shareholder Approval applicable to such issuance as soon as is possible, but in
any event not later than the 60th day after such request unless the Company has
previously used its best efforts to, but has failed to, obtain such approval
(provided, that if the Company shall fail to obtain the Shareholder Approval
during such 60-day period, the Holder may demand the cash payment set forth in
Section 10(ii) herein) or (ii) pay cash to such Holder, within five (5) Business
Days of such Holder's notice, in an amount equal to the Non-Exercise Prepayment
Amount for such Holder's portion of the Excess Warrant Shares. The payment of
the Non- Exercise Prepayment Amount to each Holder pursuant to this Section
shall be determined on a pro rata basis upon the number of Underlying Shares
issuable upon exercise of the Warrants held by such Holder on the Determination
Date which is in excess of the pro rata allocation of the Issuable Maximum. If
the Company fails to pay the Non-Exercise Prepayment Amount in full pursuant to
this Section within five (5) Business Days after the date payable, the Company
will pay interest thereon at a rate of 20% per annum to the converting Holder,
accruing interest daily from the date of exercise until such amount, plus all
such interest thereon, if any, is paid in full. Until the Company has received
the Shareholder Approval no Holder of the Warrants shall be issued, upon
exercise of Warrants, shares of Common Stock in an amount greater than such
Holder's allocated portion of the Issuable Maximum pursuant to this Section 10.

                                       10

<PAGE>

         11. Registration Rights. The Company will undertake the registration of
the Common Stock into which such Warrants are exercisable at such times and upon
such terms pursuant to the provisions of the Registration Rights Agreement.

         12. Reservation of Underlying Shares; Listing. The Company covenants
that it will at all times reserve and keep available out of its authorized
shares of Common Stock, free from preemptive rights, solely for the purpose of
issue upon exercise of the Warrants as herein provided, such number of shares of
the Common Stock as shall then be issuable upon the exercise of all outstanding
Warrants into Common Stock. The Company covenants that all shares of the Common
Stock issued upon exercise of the Warrant which shall be so issuable shall, when
issued, be duly and validly issued and fully paid and non-assessable. The
Company shall promptly secure the listing of the shares of Common Stock issuable
upon exercise of the Warrant upon each national securities exchange or automated
quotation system, if any, upon which shares of Common Stock are then listed
(subject to official notice of issuance upon exercise of this Warrant) and shall
maintain, so long as any other shares of Common Stock shall be so listed, such
listing of all shares of Common Stock from time to time issuable upon the
exercise of this Warrant; and the Company shall so list on each national
securities exchange or automated quotation system, as the case may be, and shall
maintain such listing of, any other shares of capital stock of the Company
issuable upon the exercise of this Warrant if and so long as any shares of the
same class shall be listed on such national securities exchange or automated
quotation system.

         13. Notices. Any notice or other communication required or permitted to
be given hereunder shall be in writing and shall be deemed to have been received
(a) upon hand delivery (receipt acknowledged) or delivery by telex (with correct
answer back received), telecopy or facsimile (with transmission confirmation
report) at the address or number designated below (if received by 5:00 p.m.
eastern time where such notice is to be received), or the first Business Day
following such delivery (if received after 5:00 p.m. eastern time where such
notice is to be received) or (b) on the second Business Day following the date
of mailing by express courier service, fully prepaid, addressed to such address,
or upon actual receipt of such mailing, whichever shall first occur. The
addresses for such communications are (i) if to the Company to Boston Life
Sciences, Inc., 137 Newbury Street, 8th Floor, Boston, MA 02166, Attn: Joseph
Hernon, Chief Financial Officer, fax no. (617) 425-0996 with copies to Steven A.
Wilcox, Ropes & Gray, One International Place, Boston, MA 02110, fax: (617)
951-1050 and (ii) if to the Registered Owner to the address set forth on
Schedule I to the Purchase Agreement with copies to Akin, Gump, Strauss, Hauer &
Feld, L.L.P., 590 Madison Avenue, New York, New York 10022, Attn: James Kaye,
fax no. (212) 872-1002 or such other address as may be designated in writing
hereafter, in the same manner, by such person.

         14. Compliance With Governmental Requirements. The Company covenants
that if any shares of Common Stock required to be reserved for purposes of
exercise of Warrants hereunder require registration with or approval of any
governmental authority under any Federal or state law, or any national
securities exchange, before such shares may be issued upon exercise, the Company
will use its best efforts to cause such shares to be duly registered or
approved, as the case may be.

         15. Fractional Shares. Upon any exercise hereunder, the Company shall
not be required to issue stock certificates representing fractions of shares of
the Common Stock, but may if otherwise permitted make a cash payment in respect
of any final fraction of a share based on the Per Share Market Value at such
time. If the Company elects not, or is unable, to make such a cash payment, the
Registered Owner shall be entitled to receive, in lieu of the final fraction of
a share, one whole share of Common Stock.

         16. Payment of Tax Upon Issue of Transfer. The issuance of certificates
for shares of the Common Stock upon exercise of the Warrants shall be made
without charge to the Registered Owners thereof for any documentary stamp or
similar taxes that may be payable in respect of the issue or delivery of such
certificate, provided that the Company shall not be required to pay any tax that
may be payable in respect of any transfer involved in the issuance and delivery
of any such certificate upon exercise in a name other than that of the
Registered Owner of such Warrant so converted and the Company shall not be

                                       11

<PAGE>

required to issue or deliver such certificates unless or until the person or
persons requesting the issuance thereof shall have paid to the Company the
amount of such tax or shall have established to the satisfaction of the Company
that such tax has been paid.

         17. Warrants Owned by Company Deemed Not Outstanding. In determining
whether the holders of the outstanding Warrants have concurred in any direction,
consent or waiver under this Warrant, warrants which are owned by the Company or
any other obligor on the warrants or by any person directly or indirectly
controlling or controlled by or under direct or indirect common control with the
Company or any other obligor on the warrants shall be disregarded and deemed not
to be outstanding for the purpose of any such determination; provided that any
Warrants owned by the Purchasers (as defined in the Purchase Agreement) shall be
deemed outstanding for purposes of making such a determination. Warrants so
owned which have been pledged in good faith may be regarded as outstanding if
the pledgee establishes to the satisfaction of the Company the pledgee's right
so to act with respect to such warrants and that the pledgee is not the Company
or any other obligor upon the securities or any Affiliate of the Company or any
other obligor on the warrants.

         18. Effect of Headings. The section headings herein are for convenience
only and shall not affect the construction hereof.

         19. No Rights as Stockholder. This Warrant shall not entitle the
Registered Owner to any rights as a stockholder of the Company, including
without limitation, the right to vote, to receive dividends and other
distributions, or to receive notice of, or to attend, meetings of stockholders
or any other proceedings of the Company, unless and to the extent converted into
shares of Common Stock in accordance with the terms hereof.

         20. Certain Actions Prohibited. The Company will not, by amendment of
its charter or through any reorganization, transfer of assets, consolidation,
merger, dissolution, issue or sale of securities, or any other voluntary action,
avoid or seek to avoid the observance or performance of any of the terms to be
observed or performed by it hereunder, but will at all times in good faith
assist in the carrying out of all the provisions of this Warrant and in the
taking of all such action as may reasonably be requested by the holder of this
Warrant in order to protect the exercise privilege of the holder of this Warrant
against dilution or other impairment, consistent with the tenor and purpose of
this Warrant. Without limiting the generality of the foregoing, the Company (i)
will not increase the par value of any shares of Common Stock receivable upon
the exercise of this Warrant above the Exercise Price then in effect, and (ii)
will take all such actions as may be necessary or appropriate in order that the
Company may validly and legally issue fully paid and nonassessable shares of
Common Stock upon the exercise of this Warrant.

         21. Shareholder Rights Plan. Notwithstanding the foregoing, in the
event that the Company shall distribute "poison pill" rights pursuant to a
"poison pill" shareholder rights plan (the "Rights"), the Company shall, in lieu
of making any adjustment pursuant to Section 6 hereof, make proper provision so
that each Registered Owner who exercises a Warrant after the record date for
such distribution and prior to the expiration or redemption of the Rights shall
be entitled to receive upon such exercise, in addition to the shares of Common
Stock issuable upon such exercise, a number of Rights to be determined as
follows: (i) if such exercise occurs on or prior to the date for the
distribution to the holders of Rights of separate certificates evidencing such
Rights (the "Distribution Date"), the same number of Rights to which a holder of
a number of shares of Common Stock equal to the number of shares of Common Stock
issuable upon such exercise at the time of such exercise would be entitled in
accordance with the terms and provisions of and applicable to the Rights; and
(ii) if such exercise occurs after the Distribution Date, the same number of
Rights to which a holder of the number of shares into which the Warrant to
exercised was exercisable immediately prior to the Distribution Date would have
been entitled on the Distribution Date in accordance with the terms and
provisions of and applicable to the Rights, and in each case subject to the
terms and conditions of the Rights.

                                       12

<PAGE>

         22. Successors and Assigns. This Warrant shall be binding upon and
inure to the benefit of the Registered Owners and its assigns, and shall be
binding upon any entity succeeding to the Company by merger or acquisition of
all or substantially all the assets of the Company. The Company may not assign
this Warrant or any rights or obligations hereunder without the prior written
consent of the holders of at least 60% of the Underlying Shares issuable upon
exercise of the Warrants. The Registered Owner may not assign this Warrant
without the prior written consent of the Company.

         23. Governing Law. This Warrant shall be governed by and construed and
enforced in accordance with the internal laws of the State of New York without
regard to the principles of conflicts of law thereof. Each party hereby
irrevocably submits to the nonexclusive jurisdiction of the state and federal
courts sitting in the City of New York, Borough of Manhattan, for the
adjudication of any dispute hereunder or in connection herewith or with any
transaction contemplated hereby or discussed herein, and hereby irrevocably
waives, and agrees not to assert in any suit, action or proceeding, any claim
that it is not personally subject to the jurisdiction of any such court, that
such suit, action or proceeding is improper. Each party hereby irrevocably
waives personal service of process and consents to process being served in any
such suit, action or proceeding by mailing a copy thereof to such party at the
address for such notices to it under this Warrant and agrees that such service
shall constitute good and sufficient service of process and notice thereof.
Nothing contained herein shall be deemed to limit in any way any right to serve
process in any manner permitted by law. EACH PARTY HEREBY IRREVOCABLY WAIVES ANY
RIGHT IT MAY HAVE, AND AGREES NOT TO REQUEST, A JURY TRIAL FOR THE ADJUDICATION
OF ANY DISPUTE HEREUNDER OR IN CONNECTION HEREWITH OR ARISING OUT OF THIS
AGREEMENT OR ANY TRANSACTION CONTEMPLATED HEREBY.

                                       13

<PAGE>

         IN WITNESS WHEREOF, the Company has caused this Warrant to be executed
by its duly authorized officer as of the date first set forth above.

                                            BOSTON LIFE SCIENCES, INC.

                                            By: /s/ Joseph P. Hernon
                                                -------------------------------

                                            Name: Joseph P. Hernon
                                                  -----------------------------

                                            Title: Chief Financial Officer
                                                   ----------------------------


<PAGE>

                                    EXHIBIT A

                              Warrant Exercise Form
                              ---------------------

TO:      BOSTON LIFE SCIENCES, INC.

         The undersigned hereby: (1) irrevocably subscribes for and offers to
purchase _______ shares of Common Stock of Boston Life Sciences, Inc., pursuant
to Warrant No. ___ heretofore issued to ___________________ on ____________,
199__; (2) encloses either (a) a cash payment of $__________ of (b) a Warrant
representing _____ shares of Common Stock valued at the Per Share Market Price
of $ _____ on ________, 199_, for these shares at a price of $____ per share (as
adjusted pursuant to the provisions of the Warrant); and (3) requests that a
certificate for the shares be issued in the name of the undersigned and
delivered to the undersigned at the address specified below.

    Date:                                       ________________________________

    Investor Name:                              ________________________________

    Taxpayer Identification Number:             ________________________________

    By:                                         ________________________________

    Printed Name:                               ________________________________

    Title:                                      ________________________________

    Address:                                    ________________________________


    Cashless Exercise (Y or N)  ___________



    Note:    The above signature should correspond exactly with the name on the
             face of this Warrant Certificate or with the name of assignee
             appearing in assignment form below.

AND, if said number of shares shall not be all the shares purchasable under the
within Warrant, a new Warrant Certificate is to be issued in the name of said
undersigned for the balance remaining of the shares purchasable thereunder less
any fraction of a share paid in cash and delivered to the address stated above.

<PAGE>

                             FORM OF CLASS B WARRANT

NEITHER THIS WARRANT NOR THE SECURITIES FOR WHICH THIS WARRANT IS EXERCISABLE
HAVE BEEN REGISTERED WITH THE SECURITIES AND EXCHANGE COMMISSION OR THE
SECURITIES COMMISSION OF ANY STATE IN RELIANCE UPON EXEMPTIONS FROM REGISTRATION
UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT") AND
APPLICABLE STATE LAWS, AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD EXCEPT
PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OR
PURSUANT TO AN AVAILABLE EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE
REGISTRATION REQUIREMENTS OF THE SECURITIES ACT AND IN ACCORDANCE WITH
APPLICABLE STATE SECURITIES LAWS.

IN ADDITION, THE WARRANTS REPRESENTED HEREBY ARE SUBJECT TO CERTAIN RESTRICTIONS
ON TRANSFER AND CONVERSION SET FORTH IN THE SECURITIES PURCHASE AGREEMENT DATED
AS OF SEPTEMBER 22, 1999 AMONG THE COMPANY AND THE PURCHASERS PARTY THERETO, A
COPY OF WHICH IS ON FILE IN THE OFFICE OF THE COMPANY.

September 22, 1999
775,437 shares                                         Warrant No. 2002-Reset B

                           BOSTON LIFE SCIENCES, INC.
                             STOCK PURCHASE WARRANT

Registered Owner: Brown Simpson Partners I, Ltd.

         This certifies that, for value received, Boston Life Sciences, Inc., a
Delaware corporation, the ("Company") grants the following rights to the
Registered Owner, or assigns, of this Warrant:

         1. Issue. Upon tender (as defined in Section 5 hereof) to the Company,
the Company, within three (3) Business Days of the date thereof, shall issue to
the Registered Owner, or assigns, up to the number of shares specified in
Section 2 hereof of fully paid and nonassessable shares of Common Stock that the
Registered Owner, or assigns, is otherwise entitled to purchase.

         2. Number of Shares. The total number of shares of Common Stock that
the Registered Owner, or assigns, of this Warrant is entitled to receive upon
exercise of this Warrant (the "Warrant Shares") is 775,437 shares, subject to
adjustment from time to time as to the number and kind of securities for which
this Warrant is exercisable, all as set forth in Section 6 hereof. The Company
shall at all times reserve and hold available out of its authorized and unissued
shares of Common Stock or other securities, as the case may be, sufficient
shares of Common Stock to satisfy all conversion and purchase rights represented
by outstanding convertible securities, options and warrants, including this
Warrant. The Company covenants and agrees that all shares of Common Stock or
other securities, as the case may be, that may be issued upon the exercise of
this Warrant shall, upon issuance, be duly and validly issued, fully paid and
nonassessable, free from all taxes, liens and charges with respect to the
purchase and the issuance of the shares, and shall not have any legend or
restrictions on resale, except as required by Section 3.2(b) of the Purchase
Agreement.

         3. Exercise Price.

            a. The initial per share exercise price of this Warrant,
         representing the price per share at which the shares of stock issuable
         upon exercise of this Warrant may be purchased, is $2.15, subject to
         adjustment from time to time pursuant to the provisions of Section 6
         hereof (the "Exercise Price").

<PAGE>


            b. At the option of the Registered Owner, the Exercise Price will be
         reduced to an amount which is equal to the sum of (x) the product of
         (i) 50% and (ii) the average of the Per Share Market Value for the
         seven (7) Trading Days immediately following the occurrence of either
         of the following events (each, a "Reset Event") and (y) the average of
         the Per Share Market Value for the seven (7) Trading Days immediately
         following the occurrence of such Reset Event:

               (i)   The Company issues shares of its Common Stock or options,
            warrants or other securities convertible or exchangeable therefor
            (other than Excluded Securities), at a price per share of Common
            Stock below the Exercise Price then in effect; or

               (ii)  If all or any portion of the principal amount of the
            Debenture (as defined in the Purchase Agreement is outstanding on
            the first anniversary of the Closing Date.

               (iii) If all or any portion of the principal amount of the
            Debenture (as defined in the Purchase Agreement) is outstanding on
            the second anniversary of the Closing Date.

In the case of a Reset Event under (i) above, (a) the Company shall notify the
Majority Holders in writing within five (5) Business Days of the date of such
event (the "Company Reset Notice") in accordance with Section 6(e), and (b) the
Reset Option will be deemed to be exercised, and the Exercise Price shall be
automatically reduced as of the date of such Reset Event unless, within five (5)
Business Days after the receipt by the Holder of the Company Reset Notice, the
Registered Holder provides the Company with written notification of its decision
not to exercise the Reset Option. In the case of a Reset Event under (ii) or
(iii) above, the Conversion Price shall be automatically reduced upon the
occurrence of such Reset Event. Notwithstanding the foregoing, in the case of a
Reset Event under (ii) above, the Exercise Price shall not be reduced below
$4.57, and in the case of a Reset Event under (iii) above, the Exercise Price
shall not be reduced below $2.28.

         4. Exercise Period. This Warrant may be exercised from the Closing Date
(as defined in the Purchase Agreement) up to and including September 22, 2004 (5
years less 1 day) (the "Exercise Period"). If not exercised during this period,
this Warrant and all rights granted under this Warrant shall expire and lapse.

         5. Tender; Issuance of Certificates.

            a. Subject to Section 16 hereof, this Warrant may be exercised, in
         whole or in part, by (i) actual delivery of (a) the Exercise Price in
         cash, (b) a duly executed Warrant Exercise Form, a copy of which is
         attached to this Warrant as Exhibit A, properly executed by the
         Registered Owner, or assigns, of this Warrant, and (c) by surrender of
         this Warrant, or (ii) if the resale of the Warrant Shares by the
         Registered Owner is not then registered pursuant to an effective
         registration statement under the Securities Act, delivery to the
         Company of a written notice of an election to effect a "Cashless
         Exercise" in accordance with Section 5(b), below, for the Warrant
         Shares specified in the Warrant Exercise Form. The Warrant Shares so
         purchased shall be deemed to be issued to the Registered Owner as of
         the close of business on the date on which this Warrant shall have been
         surrendered, the completed Warrant Exercise Form shall have been
         delivered and payment shall have been made for such shares as set forth
         above. The payment and Warrant Exercise Form must be delivered to the
         registered office of the Company either in person or as set for in
         Section 13 hereof.

            b. Commencing one hundred (100) days from the Filing Date (as
         defined in the Registration Rights Agreement), if, and only if, at the
         time of exercise of this Warrant, the Warrant Shares are not saleable
         pursuant to an effective registration statement, other than as allowed
         in Section 3(r) of the Registration Rights Agreement, then in addition
         to the exercise of all or a part of this Warrant by payment of the
         Exercise Price in cash as provided above, and in lieu of such payment,
         the Registered Owner shall have the right to effect a cashless exercise
         (a


                                        2

<PAGE>

         "Cashless Exercise"). In the event of a Cashless Exercise the
         Registered Owner may exercise this Warrant in whole or in part by
         surrendering this Warrant in exchange for the number of shares of
         Common Stock equal to the product of (x) the number of shares as to
         which this Warrant is being exercised multiplied by (y) a fraction, the
         numerator of which is the Per Share Market Value of the Common Stock
         less the Exercise Price then in effect and the denominator of which is
         the Per Share Market Value (in each case adjusted for fractional shares
         as herein provided).

            c. In lieu of physical delivery of the Warrant Shares, provided the
         Company's transfer agent is participating in the Depositary Trust
         Company ("DTC") Fast Automated Securities Transfer ("FAST") program,
         upon consent of the Registered Owner and in compliance with the
         provisions hereof, the Company may, at its option, use its best efforts
         to cause its transfer agent to electronically transmit the Warrant
         Shares to the Registered Owner by crediting the account of the
         Registered Owner's Prime Broker with DTC through its Deposit Withdrawal
         Agent Commission system. The time period for delivery described herein
         shall apply to the electronic transmittals described in subparagraph
         (d) below.

            d. Certificates for the Warrant Shares so purchased, representing
         the aggregate number of shares specified in the Warrant Exercise Form,
         and any cash payments due under Section 15 hereof shall be delivered to
         the Registered Owner within a reasonable time, not exceeding three (3)
         Business Days, after this Warrant shall have been so exercised. The
         certificates so delivered shall be in such denominations as may be
         requested by the Registered Owner and shall be registered in the name
         of the Registered Owner or such other name as shall be designated by
         such Registered Owner. If this Warrant shall have been exercised only
         in part, then, unless this Warrant has expired, the Company shall, at
         its expense, at the time of delivery of such certificates, deliver to
         the Registered Owner a new Warrant representing the number of shares
         with respect to which this Warrant shall not then have been exercised.

         6. Adjustment of Exercise Price.

            a. Common Stock Dividends; Common Stock Splits; Reverse Common Stock
         Splits. If the Company, at any time while this Warrant is outstanding,
         (a) shall pay a stock dividend on its Common Stock, (b) subdivide
         outstanding shares of Common Stock into a larger number of shares, (c)
         combine outstanding shares of Common Stock into a smaller number of
         shares or (d) issue by reclassification of shares of Common Stock any
         shares of capital stock of the Company, then the Exercise Price shall
         be multiplied by a fraction the numerator of which shall be the number
         of shares of Common Stock (excluding treasury shares, if any)
         outstanding before such event and the denominator of which shall be the
         number of shares of Common Stock outstanding after such event. Any
         adjustment made pursuant to this paragraph (6)(a) shall become
         effective immediately after the record date for the determination of
         shareholders entitled to receive such dividend or distribution and
         shall become effective immediately after the effective date in the case
         of a subdivision, combination or re-classification.

            b. Rights; Options; Warrants or Other Securities. If the Company, at
         any time while this Warrant is outstanding, shall issue rights,
         options, warrants or other securities to all of the holders of Common
         Stock entitling them to subscribe for or purchase, convert to, exchange
         for or otherwise acquire shares of Common Stock for no consideration or
         at a price per share less than the Threshold Price, the Exercise Price
         shall be multiplied by a fraction, the denominator of which shall be
         the number of shares of Common Stock (excluding treasury shares, if
         any) outstanding on the date of issuance of such rights, options,
         warrants or other securities plus the number of additional shares of
         Common Stock offered for subscription, purchase, conversion, exchange
         or acquisition and the numerator of which shall be the number of shares
         of Common Stock (excluding treasury shares, if any) outstanding on the
         date of issuance of such rights,


                                        3

<PAGE>

         options, warrants or other securities plus the number of shares which
         the aggregate offering price of the total number of shares so offered
         would purchase at the Exercise Price. Such adjustment shall be made
         whenever such rights, options, warrants or other securities are issued,
         and shall become effective immediately after the issue date of such
         rights, options, warrants or other securities. However, upon the
         expiration of any rights, warrants, options or other securities, the
         issuance of which resulted in an adjustment in the Exercise Price
         pursuant to this Section 6(b), if any such rights, warrants, options or
         other securities shall expire and all or any portion thereof shall not
         have been exercised, the Exercise Price shall immediately upon such
         expiration be re-computed and effective immediately upon such
         expiration be increased to the price which it would have been (but
         reflecting any other adjustments in the Exercise Price made pursuant to
         the provisions of Section 6(h) after the issuance of such rights,
         warrants, options or other securities) had the adjustment of the
         Exercise Price made upon the issuance of such rights, warrants, options
         or other securities been made on the basis of the issuance of only that
         number of shares of Common Stock (if any) actually purchased upon the
         exercise of such rights, warrants, options or other securities actually
         exercised.

            c. Subscription Rights. If the Company, at any time while this
         Warrant is outstanding, shall distribute to all of the holders of
         Common Stock evidence of its indebtedness or assets or rights, options,
         warrants or other security entitling them to subscribe for or purchase,
         convert to, exchange for or otherwise acquire any security (excluding
         those referred to in paragraphs 6(a) and (b) above), then in each such
         case the Exercise Price at which the Warrant shall thereafter be
         exercisable shall be determined by multiplying the Exercise Price in
         effect immediately prior to the date of such distribution by a
         fraction, the denominator of which shall be the Per Share Market Value
         of Common Stock determined as of the record date mentioned above, and
         the numerator of which shall be such Per Share Market Value of the
         Common Stock on such date less the then fair market value at such
         record date of the portion of such assets or evidence of indebtedness
         so distributed applicable to one outstanding share of Common Stock as
         determined by the Board of Directors in good faith; provided, however,
         that in the event of a distribution exceeding ten percent (10%) of the
         net assets of the Company, such fair market value shall be determined
         by an Appraiser selected in good faith by the Registered Owner of the
         Warrant; and provided, further, that the Company, after receipt of the
         determination by such Appraiser shall have the right to select in good
         faith an additional Appraiser meeting the same qualifications in which
         case the fair market value shall be equal to the average of the
         determinations by each such Appraiser. Such adjustment shall be made
         whenever any such distribution is made and shall become effective
         immediately thereafter.

            d. Rounding. All calculations under this Section 6 shall be made to
         the nearest cent or the nearest l/l00th of a share, as the case may be.

            e. Notice of Adjustment. Whenever the Exercise Price is adjusted
         pursuant to paragraphs 3(b) or 6(a), (b), (c) or (h), the Company shall
         promptly deliver to the Registered Owner a notice setting forth the
         Exercise Price after such adjustment and setting forth a brief
         statement of the facts requiring such adjustment.

            f. Reclassification, Etc. In the event of any reclassification of
         the Common Stock or any compulsory share exchange pursuant to which the
         Common Stock is converted into other securities, cash or property, the
         Registered Owner shall have the right thereafter to convert the shares
         of Common Stock underlying this Warrant into the securities, cash or
         property receivable upon or deemed to be held by holders of Common
         Stock following such reclassification or compulsory share exchange, and
         the Registered Owner shall be entitled upon such event to receive such
         amount of securities, cash or property received by a holder of Common
         Stock in an amount equal to the difference of (i) the amount such
         holder would have received had he held the number of shares of Common
         Stock for which this Warrant could have been exercised


                                        4

<PAGE>

         immediately prior to such reclassification or compulsory share exchange
         minus (ii) the Exercise Price in effect immediately prior to such date,
         multiplied by the number of shares of Common Stock for which this
         Warrant could have been exercised immediately prior to such
         reclassification or compulsory share exchange.

            g. Notice of Certain Events. If:

               (i)   the Company shall declare a dividend (or any other
            distribution) on its Common Stock; or

               (ii) the Company shall declare a special nonrecurring cash
            dividend on or a redemption of its Common Stock; or

               (iii) the Company shall authorize the granting to the holders of
            the Common Stock rights or warrants to subscribe for or purchase
            any shares of capital stock of any class or of any rights; or

               (iv) the approval of any shareholders of the Company shall be
            required in connection with any reclassification of the Common Stock
            of the Company, any consolidation or merger to which the Company is
            a party, any sale or transfer of all or substantially all of the
            assets of the Company, or any compulsory share exchange whereby the
            Common Stock is converted into other securities, cash or property;
            or

               (v) the Company shall authorize the voluntary or involuntary
            dissolution, liquidation or winding up of the affairs of the
            Company;

then the Company shall cause to be filed at each office or agency maintained for
the purpose of exercise of this Warrant, and shall cause to be delivered to the
Registered Owner, at least 10 Business Days prior to the applicable record or
effective date hereinafter specified, a notice (provided such notice shall not
include any material non-public information) stating (x) the date on which a
record is to be taken for the purpose of such dividend, distribution,
redemption, rights or warrants, or if a record is not to be taken, the date as
of which the holders of Common Stock of record to be entitled to such dividend,
distributions, redemption, rights or warrants are to be determined or (y) the
date on which such reclassification, consolidation, merger, sale, transfer or
share exchange is expected to become effective or close, and the date as of
which it is expected that holders of Common Stock of record shall be entitled to
exchange their shares of Common Stock for securities, cash or other property
deliverable upon such reclassification, consolidation, merger, sale, transfer or
share exchange; provided, however, that the failure to mail such notice or any
defect therein or in the mailing thereof shall not affect the validity of the
corporate action required to be specified in such notice.

            h. Adjustment to Exercise Price. If the Company, at any time while
         this Warrant is outstanding, takes any of the actions described in this
         Section 6(h), then, in order to prevent dilution of the rights granted
         under this Warrant, the Exercise Price will be subject to adjustment
         from time to time as provided in this Section 6(h); provided, however,
         that no adjustment to the Conversion Price shall be made pursuant to
         this Section 6(h) in respect of any action by the Company with respect
         to which the Conversion Price is adjusted pursuant to Section 3(b)
         above.

               (i) Adjustment of Exercise Price upon Issuance of Common Stock.
            If at any time while this Warrant is outstanding the Company issues
            or sells, or is deemed to have issued or sold, any shares of Common
            Stock (other than any Excluded Securities) for a consideration per
            share less than the lower of the Per Share Market Value on the
            Trading Day immediately prior to the date of issuance or sale and
            (y) the Exercise Price (such lesser price, the "Threshold Price"),
            the Exercise Price then in effect shall be reduced to an amount
            equal to the consideration per share of Common Stock of such
            issuance or


                                        5

<PAGE>

         sale. For the purpose of determining the adjusted Exercise Price under
         this Section 6(h)(i), the following shall be applicable:

            (A) Issuance of Options. If at any time while this Warrant is
         outstanding the Company in any manner grants, issues or sells any
         rights, options, warrants, options to subscribe for or to purchase
         Common Stock or any stock or other securities convertible into or
         exchangeable for Common Stock (other than any Excluded Securities)
         (such rights, option or warrants being herein called "Options" and such
         convertible or exchangeable stock or securities being herein called
         "Convertible Securities") and the price per share for which Common
         Stock is issuable upon the exercise of such Options or upon conversion
         or exchange of such Convertible Securities is less than the Threshold
         Price, then the Exercise Price shall be adjusted to equal the price per
         share for which Common Stock is issuable upon the exercise of such
         Options or upon the conversion or exchange of such Convertible
         Securities. No adjustment of the Exercise Price shall be made upon the
         actual issuance of such Common Stock or of such Convertible Securities
         upon the exercise of such Options or upon the actual issuance of such
         Common Stock upon conversion or exchange of such Convertible
         Securities.

            (B) Issuance of Convertible Securities. If at any time while this
         Warrant is outstanding the Company in any manner issues or sells any
         Convertible Securities and the price per share for which Common Stock
         is issuable upon such conversion or exchange (other than any Excluded
         Securities) is less than the Threshold Price, then the Exercise Price
         shall be adjusted to equal the price per share for which Common Stock
         is issuable upon the conversion or exchange of such Convertible
         Securities.

            (C) Change in Option Price or Rate of Conversion. If there is a
         change at any time in (i) the purchase price provided for in any
         Options, (ii) the additional consideration, if any, payable upon the
         issuance, conversion or exchange of any Convertible Securities or (iii)
         the rate at which any Convertible Securities are convertible into or
         exchangeable for Common Stock, then the Exercise Price in effect at the
         time of such change shall be readjusted to the Exercise Price which
         would have been in effect at such time had such Options or Convertible
         Securities still outstanding provided for such changed purchase price,
         additional consideration or changed conversion rate, as the case may
         be, at the time initially granted, issued or sold; provided that no
         adjustment shall be made if such adjustment would result in an increase
         of the Exercise Price then in effect.

            (D) Effect on Exercise Price of Certain Events. For purposes of
         determining the adjusted Exercise Price under this Section 6(h)(i), the
         following shall be applicable:

                (I) Calculation of Consideration Received. If any Common Stock,
            Options or Convertible Securities are issued or sold or deemed to
            have been issued or sold for cash, the consideration received
            therefor will be deemed to be the net amount received by the Company
            therefor. In case any Common Stock, Options or Convertible
            Securities are issued or sold for a consideration other than cash,
            the amount of the consideration other than cash received by the
            Company will be the fair value of such consideration, except where
            such consideration consists of securities, in which case the amount
            of consideration received by the Company in respect of such
            securities will be based on the Average Price of such securities on
            the Trading Day immediately preceding the date of receipt thereof.
            In case any Common Stock, Options or Convertible Securities are
            issued to the owners of the non-surviving entity in connection with
            any merger in which the Company is the surviving entity, the amount
            of consideration therefor will be deemed to be the fair value of
            such portion of the net assets and business of the


                                        6

<PAGE>

            non-surviving entity as is attributable to such Common Stock,
            Options or Convertible Securities, as the case may be. The fair
            value of any consideration other than cash or securities will be
            determined jointly by the Company and the registered owners of at
            least 60% of the Underlying Shares and Warrants then outstanding. If
            such parties are unable to reach agreement within ten (10) days
            after the occurrence of an event requiring valuation (the "Valuation
            Event"), the fair value of such consideration will be determined
            within forty-eight (48) hours of the tenth (10th) day following the
            Valuation Event by an Appraiser selected in good faith by the
            Company and agreed upon in good faith by the holders of at least 60%
            of the Underlying Shares and the Warrants then outstanding. The
            determination of such Appraiser shall be binding upon all parties
            absent manifest error.

                (II)  Integrated Transactions. In case any Option is issued in
            connection with the issue or sale of other securities of the
            Company, together comprising one integrated transaction in which no
            specific consideration is allocated to such Options by the parties
            thereto, the Options will be deemed to have been issued for an
            aggregate consideration of $ .01.

                (III) Treasury Shares. The number of shares of Common Stock
            outstanding at any given time does not include shares owned or held
            by or for the account of the Company, and the disposition of any
            shares so owned or held will be considered an issue or sale of
            Common Stock.

                (IV)  Record Date. If the Company takes a record of the holders
            of Common Stock for the purpose of entitling them (1) to receive a
            dividend or other distribution payable in Common Stock, Options or
            in Convertible Securities or (2) to subscribe for or purchase Common
            Stock, Options or Convertible Securities, then such record date will
            be deemed to be the date of the issue or sale of the shares of
            Common Stock deemed to have been issued or sold upon the declaration
            of such dividend or the making of such other distribution or the
            date of the granting of such right of subscription or purchase, as
            the case may be.

                (V)   Certain Events. If any event that would adversely affect
            the rights of the Registered Owner of this Warrant occurs but is not
            expressly provided for by Section 6 hereof (including, without
            limitation, the granting of stock appreciation rights, phantom stock
            rights or other rights with equity features), then the Company's
            Board of Directors will make an appropriate adjustment in the
            Exercise Price so as to protect the rights of the Registered Owner
            of this Warrant; provided, however, that no such adjustment will
            increase the Exercise Price as otherwise determined pursuant to this
            Section 6(h).

         Notwithstanding the foregoing, in no event shall any provision in this
Section 6 cause the Exercise Price to be greater than the Exercise Price on the
date of issuance of this Warrant.

         i. Adjustment of Number of Shares. Upon each adjustment of the Exercise
Price as a result of the calculations made in this Section 6, the number of
Warrant Shares shall be multiplied by a fraction, the numerator of which shall
be the number of shares of Common Stock (excluding treasury shares, if any)
outstanding after such event and the denominator of which shall be the number of
shares of Common Stock (excluding treasury shares, if any) outstanding before
such event.


                                        7

<PAGE>

         7. Restriction on Conversion by Either the Registered Owner or the
Company. Notwithstanding anything herein to the contrary, in no event shall any
Registered Owner or the Company have the right or be required to exercise this
Warrant if as a result of such conversion the aggregate number of shares of
Common Stock beneficially owned by such Registered Owner and its Affiliates
would exceed 9.99% of the outstanding shares of the Common Stock following such
exercise. For purposes of this Section 7, beneficial ownership shall be
calculated in accordance with Section 13(d) of the Securities Exchange Act of
1934, as amended. The provisions of this Section 7 may be waived by a Registered
Owner as to itself (and solely as to itself) upon not less than 65 days prior
written notice to the Company, and the provisions of this Section 7 shall
continue to apply until such 65th day (or later, if stated in the notice of
waiver).

         8. Officer's Certificate. Whenever the number of shares purchasable
upon exercise shall be adjusted as required by the provisions of Section 3(b) or
Section 6, the Company shall forthwith file in the custody of its Secretary or
an Assistant Secretary at its principal office and with its stock transfer
agent, if any, an officer's certificate showing the adjusted Exercise Price,
number of shares or other securities determined as herein provided, setting
forth in reasonable detail the facts requiring such adjustment and the manner of
computing such adjustment. Each such officer's certificate shall be signed by
the chairman, president or chief financial officer of the Company and by the
secretary or any assistant secretary of the Company. Each such officer's
certificate shall be made available at all reasonable times for inspection by
any Registered Owner of the Warrants and the Company shall, forthwith after each
such adjustment, deliver a copy of such certificate to the each of the
Registered Owners.

         9. Definitions. Capitalized terms used herein and not otherwise defined
herein shall have the meanings given to such terms in the Purchase Agreement. As
used in this Warrant, the following terms have the following meanings:

         "Affiliate" means, with respect to any Person, any other Person that
directly or indirectly controls or is controlled by or under common control with
such Person. For the purposes of this definition, "control," when used with
respect to any Person, means the possession, direct or indirect, of the power to
direct or cause the direction of the management and policies of such Person,
whether through the ownership of voting securities, by contract or otherwise;
and the terms of "affiliated," "controlling" and "controlled" have meanings
correlative to the foregoing.

         "Appraiser" shall mean a nationally recognized or major regional
investment banking firm or firm of independent certified public accountants of
recognized standing.

         "Approved Stock Plan" shall mean any contract, plan or agreement which
has been approved by the Board of Directors of the Company, pursuant to which
the Company's securities may be issued to any employee, officer, director or
consultant or other person with a business relationship with the Company.

         "Business Day" means any day except Saturday, Sunday and any day which
shall be a legal holiday or a day on which banking institutions in the state of
New York generally are authorized or required by law or other government actions
to close.

         "Change of Control" has the meaning set forth in Section 4.1 of the
Purchase Agreement.

         "Closing" has the meaning set forth in Section 1.2 of the Purchase
Agreement.

         "Common Stock" means the shares of the Company's Common Stock, par
value $0.01 per share.

         "Company" means Boston Life Sciences, Inc., a Delaware corporation.

         "Convertible Securities" has the meaning assigned to it in Section
6(h)(i)(A) hereof.


                                        8

<PAGE>

         "Excluded Securities" means (i) the Debentures, the Warrants and the
Underlying Shares, (ii) shares of Common Stock of the Company, or any options,
warrants or other securities convertible or exchangeable therefor, that are
issued or deemed to have been issued from time to time in connection with an
Approved Stock Plan, (iii) shares of Common Stock of the Company issuable upon
the exercise of any options, warrants or other securities convertible or
exercisable therefor outstanding on the date hereof and listed in Schedule
2.1(c) of the Purchase Agreement, (iv) shares of Common Stock of the Company, or
any options, warrants or other securities convertible or exchangeable therefor,
that are issued or deemed to have been issued as consideration for an
acquisition by the Company of a division, assets or business (or stock
constituting any portion thereof) from another Person, (v) warrants for up to
200,000 shares of Common Stock per year, not exceeding 600,000 shares in the
aggregate, provided that the conversion price of any such warrants is above the
Per Share Market Value on the date the warrant is issued, (and the Common Stock
issued upon exercise thereof) issued after the date hereof by the Company for
purposes other than raising capital, (vi) either Common Stock or preferred stock
convertible into Common Stock, provided that such stock shall be issued at a
price per share of not less than $4.50 in return for proceeds not to exceed the
greater of (1) $2,000,000 or (2) the amount expended by the Company in the
redemption of the Company's Series C floating preferred stock; provided, further
however, that (a) such stock shall not be issued prior to December 1, 1999 and
resales of such Common Stock (or Common Stock issuable upon conversion of such
preferred stock) shall not be registered under the Securities Act prior to April
1, 2000, (b) the purchasers of such Common Stock shall be prohibited from
executing short sales of Common Stock under substantially the same terms and
conditions as the Purchasers are hereunder, and (c) Brown Simpson shall have a
right of first offer with respect to such sale of stock at a price that is not
less than the price at which such stock is eventually sold; and provided further
that if preferred stock is issued, the conversion terms of such preferred stock
shall be no more favorable to investors than the conversion terms in the
Debenture and (vii) warrants described on Schedule 2.1(m) to the Securities
Purchase Agreement.

         "Exercise Period" has the meaning assigned to it the Section 4 hereof.

         "Exercise Price" has the meaning assigned to it in Section 3 hereof.

         "Options" has the meaning assigned to it in Section 6(h)(i)(A) hereof.

         "Per Share Market Value" means on any particular date (i) the closing
bid price per share of the Common Stock on such date on the Nasdaq Smallcap
Market or other registered national stock exchange on which the Common Stock is
then listed or if there is no such price on such date, then the closing bid
price on such exchange or quotation system on the date nearest preceding such
date, or (ii) if the Common Stock is not listed then on the Nasdaq Smallcap
Market or any registered national stock exchange, the closing bid price for a
share of Common Stock in the over-the-counter market, as reported by the
National Quotation Bureau Incorporated (or similar organization or agency
succeeding to its functions of reporting prices) at the close of business on
such date, or (iii) if the Common Stock is not then publicly traded the fair
market value of a share of Common Stock as determined by an Appraiser selected
in good faith by the holders of at least 60% of the underlying shares issuable
upon exercise of the warrants; provided, however, that the Company, after
receipt of the determination by such Appraiser, shall have the right to select,
in good faith, an additional Appraiser, in which case the fair market value
shall be equal to the average of the determinations by each such Appraiser; and
provided, further that all determinations of the Per Share Market Value shall be
appropriately adjusted for any stock dividends, stock splits or other similar
transactions during such period.

         "Purchase Agreement" means that certain Securities Purchase Agreement,
dated September 22, 1999, among the Company and the Purchasers.

         "Purchaser" has the meaning set forth in the Purchase Agreement.


                                        9

<PAGE>

         "Registered Owner" means the person identified on the face of this
Warrant as the registered owner hereof or such other person as shown on the
records of the Company as being the registered owner of this Warrant or their
assigns.

         "Registration Rights Agreement" means that certain Registration Rights
Agreement, dated September 22, 1999, among the Company and the Purchasers.

         "Reset Event" has the meaning assigned to it in Section 3(b) hereof.

         "Threshold Price" has the meaning set forth in Section 6(h)(i) hereof.

         "Trading Day(s)" means any day on which the primary market on which
shares of Common Stock are listed is open for trading.

         "Underlying Shares" has the meaning assigned to it in Section 2.1(d) of
the Purchase Agreement.

         "Warrant(s)" means the warrants issuable at the Closing.

         10. NASDAQ Limitation. The Company covenants that if any shares of
Common Stock required to be reserved for purposes of conversion of Debentures
hereunder require registration with or approval of any governmental authority
under any Federal or state law, or any national securities exchange, before such
shares may be issued upon conversion, the Company will use its best efforts to
cause such shares to be duly registered or approved, as the case may be.

         If on any date (the "Determination Date") (a) the Common Stock is
listed for trading on the Nasdaq Small Cap Market or the Nasdaq National Market,
(b) the Exercise Price then in effect is such that the sum of (x) the aggregate
number of shares of Common Stock that would then be issuable upon conversion in
full of the then outstanding principal amount of the Debentures and exercise in
full of the Warrants as if all such Debentures were converted and such warrants
were exercised on such Determination Date (without regard to any limitations on
conversions) and as payment of interest thereon, and (y) the aggregate number of
shares of Common Stock issued to the Purchasers upon any prior conversion of the
Debentures or exercise of the Warrants, would equal or exceed 20% of the number
of shares of the Common Stock outstanding on the Closing Date (such number of
shares as would not equal or exceed such 20% limit, the "Issuable Maximum"), and
(c) the Company shall not have previously obtained the vote of the shareholders
of the Company (the "Shareholder Approval"), if any, as may be required by the
applicable rules and regulations of Nasdaq (or any successor entity) to approve
the issuance of shares of Common Stock in excess of the Issuable Maximum in a
private placement whereby shares of Common Stock are deemed to have been issued
at a price that is less than the greater of book value or fair market value of
the Common Stock, then with respect to the number of Warrant Shares the issuance
of which would result in an issuance of shares of Common Stock in excess of the
Warrant Holder's (a "Holder") pro rata allocation (as described below) of the
Issuable Maximum (the "Excess Warrant Shares"), the Company may elect to prepay
cash to the Holders in an amount equal to the product of (A) the difference
between the Per Share Market Value and the Exercise Price, multiplied by (B) the
pro rata allocation of such Holder's Excess Warrant Shares (the "Non-Exercise
Payment Amount"). Any such election by the Company must be made in writing to
the Holders within five (5) Trading Days after the Determination Date and the
payment of such Non-Exercise Prepayment Amount must be made in full to the
Holders within ten (10) Business Days after the date such notice is delivered.
If the Company does not deliver timely a notice of its election to prepay under
this Section or shall, if it shall have delivered such a notice, fail to pay the
Non-Exercise Payment Amount hereunder within ten (10) Business Days thereafter,
then the holders of at least 60% of the Underlying Shares exercisable upon
exercise of the Warrants shall have the option by written notice to the Company,
to declare any such notice given by the Company, if given, to be null and void
and require the Company to either: (i) use its best efforts to obtain the
Shareholder Approval applicable to such issuance as soon as is possible, but in
any event not later than the 60th day after such request unless the Company has
previously used its best efforts to, but has


                                       10

<PAGE>

failed to, obtain such approval (provided, that if the Company shall fail to
obtain the Shareholder Approval during such 60-day period, the Holder may demand
the cash payment set forth in Section 10(ii) herein) or (ii) pay cash to such
Holder, within five (5) Business Days of such Holder's notice, in an amount
equal to the Non-Exercise Prepayment Amount for such Holder's portion of the
Excess Warrant Shares. The payment of the Non- Exercise Prepayment Amount to
each Holder pursuant to this Section shall be determined on a pro rata basis
upon the number of Underlying Shares issuable upon exercise of the Warrants held
by such Holder on the Determination Date which is in excess of the pro rata
allocation of the Issuable Maximum. If the Company fails to pay the Non-Exercise
Prepayment Amount in full pursuant to this Section within five (5) Business Days
after the date payable, the Company will pay interest thereon at a rate of 20%
per annum to the converting Holder, accruing interest daily from the date of
exercise until such amount, plus all such interest thereon, if any, is paid in
full. Until the Company has received the Shareholder Approval no Holder of the
Warrants shall be issued, upon exercise of Warrants, shares of Common Stock in
an amount greater than such Holder's allocated portion of the Issuable Maximum
pursuant to this Section 10.

         11. Registration Rights. The Company will undertake the registration of
the Common Stock into which such Warrants are exercisable at such times and upon
such terms pursuant to the provisions of the Registration Rights Agreement.

         12. Reservation of Underlying Shares; Listing. The Company covenants
that it will at all times reserve and keep available out of its authorized
shares of Common Stock, free from preemptive rights, solely for the purpose of
issue upon exercise of the Warrants as herein provided, such number of shares of
the Common Stock as shall then be issuable upon the exercise of all outstanding
Warrants into Common Stock. The Company covenants that all shares of the Common
Stock issued upon exercise of the Warrant which shall be so issuable shall, when
issued, be duly and validly issued and fully paid and non-assessable. The
Company shall promptly secure the listing of the shares of Common Stock issuable
upon exercise of the Warrant upon each national securities exchange or automated
quotation system, if any, upon which shares of Common Stock are then listed
(subject to official notice of issuance upon exercise of this Warrant) and shall
maintain, so long as any other shares of Common Stock shall be so listed, such
listing of all shares of Common Stock from time to time issuable upon the
exercise of this Warrant; and the Company shall so list on each national
securities exchange or automated quotation system, as the case may be, and shall
maintain such listing of, any other shares of capital stock of the Company
issuable upon the exercise of this Warrant if and so long as any shares of the
same class shall be listed on such national securities exchange or automated
quotation system.

         13. Notices. Any notice or other communication required or permitted to
be given hereunder shall be in writing and shall be deemed to have been received
(a) upon hand delivery (receipt acknowledged) or delivery by telex (with correct
answer back received), telecopy or facsimile (with transmission confirmation
report) at the address or number designated below (if received by 5:00 p.m.
eastern time where such notice is to be received), or the first Business Day
following such delivery (if received after 5:00 p.m. eastern time where such
notice is to be received) or (b) on the second Business Day following the date
of mailing by express courier service, fully prepaid, addressed to such address,
or upon actual receipt of such mailing, whichever shall first occur. The
addresses for such communications are (i) if to the Company to Boston Life
Sciences, Inc., 137 Newbury Street, 8/th/ Floor, Boston, MA 02166, Attn: Joseph
Hernon, Chief Financial Officer, fax no. (617) 425-0996 with copies to Steven A.
Wilcox, Ropes & Gray, One International Place, Boston, MA 02110, fax: (617)
951-1050 and (ii) if to the Registered Owner to the address set forth on
Schedule I to the Purchase Agreement with copies to Akin, Gump, Strauss, Hauer &
Feld, L.L.P., 590 Madison Avenue, New York, New York 10022, Attn: James Kaye,
fax no. (212) 872-1002 or such other address as may be designated in writing
hereafter, in the same manner, by such person.

         14. Compliance With Governmental Requirements. The Company covenants
that if any shares of Common Stock required to be reserved for purposes of
exercise of Warrants hereunder require

                                       11

<PAGE>

registration with or approval of any governmental authority under any Federal or
state law, or any national securities exchange, before such shares may be issued
upon exercise, the Company will use its best efforts to cause such shares to be
duly registered or approved, as the case may be.

         15. Fractional Shares. Upon any exercise hereunder, the Company shall
not be required to issue stock certificates representing fractions of shares of
the Common Stock, but may if otherwise permitted make a cash payment in respect
of any final fraction of a share based on the Per Share Market Value at such
time. If the Company elects not, or is unable, to make such a cash payment, the
Registered Owner shall be entitled to receive, in lieu of the final fraction of
a share, one whole share of Common Stock.

         16. Payment of Tax Upon Issue of Transfer. The issuance of certificates
for shares of the Common Stock upon exercise of the Warrants shall be made
without charge to the Registered Owners thereof for any documentary stamp or
similar taxes that may be payable in respect of the issue or delivery of such
certificate, provided that the Company shall not be required to pay any tax that
may be payable in respect of any transfer involved in the issuance and delivery
of any such certificate upon exercise in a name other than that of the
Registered Owner of such Warrant so converted and the Company shall not be
required to issue or deliver such certificates unless or until the person or
persons requesting the issuance thereof shall have paid to the Company the
amount of such tax or shall have established to the satisfaction of the Company
that such tax has been paid.

         17. Warrants Owned by Company Deemed Not Outstanding. In determining
whether the holders of the outstanding Warrants have concurred in any direction,
consent or waiver under this Warrant, warrants which are owned by the Company or
any other obligor on the warrants or by any person directly or indirectly
controlling or controlled by or under direct or indirect common control with the
Company or any other obligor on the warrants shall be disregarded and deemed not
to be outstanding for the purpose of any such determination; provided that any
Warrants owned by the Purchasers (as defined in the Purchase Agreement) shall be
deemed outstanding for purposes of making such a determination. Warrants so
owned which have been pledged in good faith may be regarded as outstanding if
the pledgee establishes to the satisfaction of the Company the pledgee's right
so to act with respect to such warrants and that the pledgee is not the Company
or any other obligor upon the securities or any Affiliate of the Company or any
other obligor on the warrants.

         18. Effect of Headings. The section headings herein are for convenience
only and shall not affect the construction hereof.

         19. No Rights as Stockholder. This Warrant shall not entitle the
Registered Owner to any rights as a stockholder of the Company, including
without limitation, the right to vote, to receive dividends and other
distributions, or to receive notice of, or to attend, meetings of stockholders
or any other proceedings of the Company, unless and to the extent converted into
shares of Common Stock in accordance with the terms hereof.

         20. Certain Actions Prohibited. The Company will not, by amendment of
its charter or through any reorganization, transfer of assets, consolidation,
merger, dissolution, issue or sale of securities, or any other voluntary action,
avoid or seek to avoid the observance or performance of any of the terms to be
observed or performed by it hereunder, but will at all times in good faith
assist in the carrying out of all the provisions of this Warrant and in the
taking of all such action as may reasonably be requested by the holder of this
Warrant in order to protect the exercise privilege of the holder of this Warrant
against dilution or other impairment, consistent with the tenor and purpose of
this Warrant. Without limiting the generality of the foregoing, the Company (i)
will not increase the par value of any shares of Common Stock receivable upon
the exercise of this Warrant above the Exercise Price then in effect, and (ii)
will take all such actions as may be necessary or appropriate in order that the
Company may validly and legally issue fully paid and nonassessable shares of
Common Stock upon the exercise of this Warrant.

                                       12

<PAGE>

         21. Shareholder Rights Plan. Notwithstanding the foregoing, in the
event that the Company shall distribute "poison pill" rights pursuant to a
"poison pill" shareholder rights plan (the "Rights"), the Company shall, in lieu
of making any adjustment pursuant to Section 6 hereof, make proper provision so
that each Registered Owner who exercises a Warrant after the record date for
such distribution and prior to the expiration or redemption of the Rights shall
be entitled to receive upon such exercise, in addition to the shares of Common
Stock issuable upon such exercise, a number of Rights to be determined as
follows: (i) if such exercise occurs on or prior to the date for the
distribution to the holders of Rights of separate certificates evidencing such
Rights (the "Distribution Date"), the same number of Rights to which a holder of
a number of shares of Common Stock equal to the number of shares of Common Stock
issuable upon such exercise at the time of such exercise would be entitled in
accordance with the terms and provisions of and applicable to the Rights; and
(ii) if such exercise occurs after the Distribution Date, the same number of
Rights to which a holder of the number of shares into which the Warrant to
exercised was exercisable immediately prior to the Distribution Date would have
been entitled on the Distribution Date in accordance with the terms and
provisions of and applicable to the Rights, and in each case subject to the
terms and conditions of the Rights.

         22. Successors and Assigns. This Warrant shall be binding upon and
inure to the benefit of the Registered Owners and its assigns, and shall be
binding upon any entity succeeding to the Company by merger or acquisition of
all or substantially all the assets of the Company. The Company may not assign
this Warrant or any rights or obligations hereunder without the prior written
consent of the holders of at least 60% of the Underlying Shares issuable upon
exercise of the Warrants. The Registered Owner may not assign this Warrant
without the prior written consent of the Company.

         23. Governing Law. This Warrant shall be governed by and construed and
enforced in accordance with the internal laws of the State of New York without
regard to the principles of conflicts of law thereof. Each party hereby
irrevocably submits to the nonexclusive jurisdiction of the state and federal
courts sitting in the City of New York, Borough of Manhattan, for the
adjudication of any dispute hereunder or in connection herewith or with any
transaction contemplated hereby or discussed herein, and hereby irrevocably
waives, and agrees not to assert in any suit, action or proceeding, any claim
that it is not personally subject to the jurisdiction of any such court, that
such suit, action or proceeding is improper. Each party hereby irrevocably
waives personal service of process and consents to process being served in any
such suit, action or proceeding by mailing a copy thereof to such party at the
address for such notices to it under this Warrant and agrees that such service
shall constitute good and sufficient service of process and notice thereof.
Nothing contained herein shall be deemed to limit in any way any right to serve
process in any manner permitted by law. EACH PARTY HEREBY IRREVOCABLY WAIVES ANY
RIGHT IT MAY HAVE, AND AGREES NOT TO REQUEST, A JURY TRIAL FOR THE ADJUDICATION
OF ANY DISPUTE HEREUNDER OR IN CONNECTION HEREWITH OR ARISING OUT OF THIS
AGREEMENT OR ANY TRANSACTION CONTEMPLATED HEREBY.

                                       13

<PAGE>

         IN WITNESS WHEREOF, the Company has caused this Warrant to be executed
by its duly authorized officer as of the date first set forth above.

                                        BOSTON LIFE SCIENCES, INC.

                                        By: /s/ Joseph P. Hernon
                                            -------------------------------

                                        Name: Joseph P. Hernon
                                              -----------------------------

                                        Title: Chief Financial Officer
                                               ----------------------------


<PAGE>

                                    EXHIBIT A

                              Warrant Exercise Form

TO:      BOSTON LIFE SCIENCES, INC.

         The undersigned hereby: (1) irrevocably subscribes for and offers to
purchase _______ shares of Common Stock of Boston Life Sciences, Inc., pursuant
to Warrant No. ___ heretofore issued to ___________________ on ____________,
199__; (2) encloses either (a) a cash payment of $__________ of (b) a Warrant
representing _____ shares of Common Stock valued at the Per Share Market Price
of $ _____ on ________, 199_, for these shares at a price of $____ per share (as
adjusted pursuant to the provisions of the Warrant); and (3) requests that a
certificate for the shares be issued in the name of the undersigned and
delivered to the undersigned at the address specified below.

              Date:                               ______________________________

              Investor Name:                      ______________________________

              Taxpayer Identification Number:     ______________________________

              By:                                 ______________________________

              Printed Name:                       ______________________________

              Title:                              ______________________________

              Address:                            ______________________________

                                                  ______________________________

                                                  ______________________________


              Cashless Exercise (Y or N)    ________



              Note:       The above signature should correspond exactly
              with the name on the face of this Warrant Certificate or
              with the name of assignee appearing in assignment form
              below.

AND, if said number of shares shall not be all the shares purchasable under the
within Warrant, a new Warrant Certificate is to be issued in the name of said
undersigned for the balance remaining of the shares purchasable thereunder less
any fraction of a share paid in cash and delivered to the address stated above.