<PAGE>

                                                                     EXHIBIT 4.3

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR
ANY STATE SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR
HYPOTHECATED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF A REGISTRATION STATEMENT
IN EFFECT WITH RESPECT TO THE SECURITIES UNDER SUCH ACT OR AN OPINION OF COUNSEL
SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED OR UNLESS
SOLD PURSUANT TO RULE 144 OF SUCH ACT. ANY SUCH TRANSFER MAY ALSO BE SUBJECT TO
APPLICABLE STATE SECURITIES LAWS.



                          BOSTON LIFE SCIENCES, INC.



                     Warrant for the Purchase of Shares of
                     -------------------------------------
                                 Common Stock
                                 ------------



No.  BLSI - [INSERT NUMBER]                      [INSERT NUMBER] Shares


FOR VALUE RECEIVED, BOSTON LIFE SCIENCES, INC., A Delaware corporation (the
"Company"), hereby certifies that [INSERT NAME] or its permitted assigns, is
entitled to purchase from the Company, at any time or from time to time,
commencing on [INSERT DATE] (the "Initial Exercise Date"). This Warrant shall
expire at 5:00 P. M., New York City time, on [INSERT DATE], and is exercisable
for [INSERT NUMBER] fully paid and non-assessable shares of the Common Stock,
$.01 par value per share, of the Company for an aggregate purchase price of
[INSERT PRICE] computed on the basis of [INSERT PRICE] per share. (Hereinafter,
(i) said Common Stock, together with any other equity securities which may be
issued by the Company with respect thereto or in substitution therefor, is
referred to as the "Common Stock", (ii) the shares of the Common Stock
purchasable hereunder or under any other Warrant (as hereinafter defined) are
referred to as the "Warrant Shares", (iii) the aggregate purchase price payable
for the Warrant Shares hereunder is referred to as the "Aggregate Warrant
Price", (iv) the price payable for each of the Warrant Shares hereunder is
referred to as the "Per Share Warrant Price", (v) this Warrant, all similar
Warrants issued on the date hereof and all warrants hereafter issued in exchange
or substitution for this Warrant or such similar Warrants are referred to as the
"Warrants" and (vi) the holder of this Warrant is referred to as the "Holder"
and the holder of this Warrant and all other Warrants or Warrant Shares issued
upon the exercise of any Warrant are referred to as the "Holders"). The
Aggregate Warrant Price is not subject to adjustment. The Per Share Warrant
Price is subject to adjustment as hereinafter provided; in the event of any such
adjustment, the number of Warrant Shares shall be adjusted by dividing the
Aggregate Warrant Price by the Per Share Warrant Price in effect immediately
after such adjustment.
<PAGE>

     1.   Exercise of Warrant
          -------------------

          (a)    This Warrant may be exercised, in whole at any time or in part
from time to time, commencing on the Initial Exercise Date and prior to 5:00 P.
M. New York City time, on the Termination Date by the holder by the surrender of
this Warrant (with the subscription form at the end hereof duly executed) at the
address set forth in Subsection 9(a) hereof, together with proper payment of the
Aggregate Warrant Price, or the proportionate part thereof if this Warrant is
exercised in part, with payment for Warrant Shares made by certified or official
bank check payable to the order of the Company.

          (b)    If this Warrant is exercised in part, this Warrant must be
exercised for a number of whole shares of the Common Stock and the Holder is
entitled to receive a new Warrant covering the Warrant Shares which have not
been exercised and setting forth the proportionate part of the Aggregate Warrant
Price applicable to such Warrant Shares.  Upon surrender of this Warrant, the
Company will (i) issue a certificate or certificates in the name of the Holder
for the largest number of whole shares of the Common Stock to which the Holder
shall be entitled and, if this Warrant is exercised in whole, in lieu of any
fractional share of the Common Stock to which the Holder shall be entitled, pay
to the Holder cash in an amount equal to the fair value of such fractional share
(determined in such reasonable manner as the Company shall determine), and (ii)
deliver the other securities and properties receivable upon the exercise of this
Warrant, if any, or the proportionate part thereof if this Warrant is exercised
in part, pursuant to the provisions of this Warrant.

     2.   Reservation of Warrant Shares; Listing.  The Company agrees that,
          --------------------------------------
prior to the expiration of this Warrant, the Company will at all times (a) have
authorized and in reserve, and will keep available, solely for issuance or
delivery upon the exercise of this Warrant, the shares of the Common Stock and
other securities and properties as from time to time shall be receivable upon
the exercise of this Warrant, free and clear of all restrictions on sale or
transfer, except for the restrictions on sale or transfer set forth in the
Securities Act of 1933, as amended (the "Act"), and restrictions created by or
on behalf of the Holder, and free and clear of all preemptive rights and rights
of first refusal; and (b) if the Company prepares and files a registration
statement covering the shares of Common Stock issued or issuable upon exercise
of this Warrant with the Securities and Exchange Commission (the "SEC") which
registration statement is declared effective by the SEC under the Act and the
Company lists its Common Stock on any national securities exchange, it will use
its best efforts to cause the shares of Common Stock subject to this Warrant to
be listed on such exchange.

     3.   Protection Against Dilution.
          ---------------------------

          (a)    In case the Company shall hereafter (i) pay a dividend or make
a distribution on its capital stock in shares of Common Stock, (ii) subdivide
its outstanding shares of Common Stock into a great number of shares, (iii)
combine its outstanding shares of Common Stock into a smaller number of shares
or (iv) issue by reclassification of its Common Stock any shares of capital
stock of the Company, the Per Share Warrant Price shall be adjusted to be equal
to a fraction, the numerator of which shall be the Aggregate Warrant Price and
the denominator of which shall be the number of shares of Common Stock or other
capital stock of the Company which he would have owned immediately following
such action had such Warrant been exercised immediately prior thereto. An
adjustment made pursuant to this Subsection shall become effective immediately
after the record date in the case of a dividend or distribution and shall become
effective immediately after the effective date in the case of a subdivision,
combination or reclassification.

                                       2
<PAGE>

          (b)    No adjustment in the Per Share Warrant Price shall be required
in the case of the issuance by the Company of (a) Common Stock pursuant to the
exercise of any Warrant, (b) options or warrants to purchase Common Stock
(including the exercise thereof) issued or sold to employees, officers or
directors of or consultants and advisers to the Company or any subsidiary
thereof, and (c) shares of Common Stock issued or sold pursuant to stock
purchase or stock option plans or other similar arrangements that are approved
by the Company's Board of Directors.

          (c)    In case of any capital reorganization or reclassification, or
any consolidation or merger to which the Company is a party other than a merger
or consolidation in which the Company is the continuing corporation, or in case
of any sale or conveyance to another entity of the property of the Company as an
entirety or substantially as an entirety, or in the case of any statutory
exchange of securities with another corporation (including any exchange effected
in connection with a merger of a third corporation into the Company), the Holder
of this Warrant shall have the right thereafter to receive on the exercise of
this Warrant the kind and amount of securities, cash or other property which the
Holder would have owned or have been entitled to receive immediately after such
reorganization, reclassification, consolidation, merger, statutory exchange,
sale or conveyance had this Warrant been exercised immediately prior to the
effective date of such reorganization, reclassification, consolidation, merger,
statutory exchange, sale or conveyance and in any such case, if necessary,
appropriate adjustment shall be made in the application of the provisions set
forth in this Section 3 with respect to the rights and interests thereafter of
the Holder of this Warrant to the end that the provisions set forth in this
Section 3 shall thereafter correspondingly be made applicable, as nearly as may
reasonably be, in relation to any shares or other securities or property
thereafter deliverable on the exercise of this Warrant. The above provisions of
this Subsection shall similarly apply to successive reorganizations,
reclassifications, consolidations, mergers, statutory exchanges, sales or
conveyances. The issuer of any shares of stock or other securities or property
thereafter deliverable on the exercise of this Warrant shall be responsible for
all of the agreements and obligations of the Company hereunder. Notice of any
such reorganization, reclassification, consolidation, merger, statutory
exchange, sale or conveyance and of said provisions so proposed to be made,
shall be mailed to the Holders of the Warrants not less than 30 days prior to
such event. A sale of all or substantially all of the assets of the Company for
a consideration consisting primarily of securities shall be deemed a
consolidation or merger for the foregoing purposes.

          (d)    No adjustment in the Per Share Warranty Price shall be required
unless such adjustment would require an increase or decrease of at least $0.05
per share of Common Stock; provided, however, that any adjustments which by
                           --------  -------
reason of this Subsection are not required to be made shall be carried forward
and taken into account in any subsequent adjustment; provided, further, however,
                                                     -------- --------
that adjustments shall be required and made in accordance with the provisions of
this Section 3 (other than this Subsection) not later than such times as may be
required in order to preserve the tax-free nature of a distribution to the
Holder of this Warrant or Common Stock issuable upon the exercise hereof.  All
calculations under this section 3 shall be made to the nearest cent or to the
nearest 1/100th of share, as the case may be.  Anything in this Section 3 to the
contrary notwithstanding, the Company shall be entitled to make such reductions
in the Per Share Warrant Price, in addition to those required by this Section 3,
as it in its discretion shall deem to be advisable in order that any stock
dividend, subdivision of shares or distribution of rights to purchase stock or
securities convertible or exchangeable for stock hereafter made by the Company
to its stockholders shall not be taxable.

          (e)    If the Board of Directors of the Company shall declare any
dividend or other distribution with respect to the Common Stock other than a
cash distribution out of earned surplus, the Company shall mail notice thereof
to the Holders of the Warrants not less than 15

                                       3
<PAGE>

days prior to the record date fixed for determining stockholders entitled to
participate in such dividend or other distribution.

          (f)    If, as a result of an adjustment made pursuant to this Section
3, the Holder of any Warrant thereafter surrendered for exercise shall become
entitled to receive shares of two or more classes of capital stock or shares of
Common Stock and other capital stock of the Company, the Board of Directors
(whose determination shall be conclusive and shall be described in a written
notice to the Holder of any Warrant promptly after such adjustment) shall
determine the allocation of the adjusted Per Share Warrant Price between or
among shares or such classes of capital stock or shares of Common Stock and
other capital stock.

     4.   Fully Paid Stock; Taxes.  The Company agrees that the shares of the
          -----------------------
Common Stock represented by each and every certificate of Warrant Shares
delivered on the exercise of this Warrant be validly issued and outstanding,
fully paid and nonassessable, and not subject to preemptive rights or rights of
first refusal, and the Company will take all such actions as may be necessary to
assure that the par value or stated value, if any, per share of the Common Stock
is at all times equal to or less than the then Per Share Warrant Price.  The
Company further covenants and agrees that it will pay, when due and payable, any
and all Federal and State stamp, original issue or similar taxes which may be
payable in respect of the issue of any Warrant Share or any certificate thereof.


     5.   Limited Transferability.  This Warrant may not be sold, transferred,
          ------------------------
assigned or hypothecated by the Holder (a) except in compliance with the
provisions of the Act and the applicable state securities "blue sky" laws, or
(b) in the case of an individual, pursuant to such individual's last will and
testament or the laws of descent and distribution, and is so transferable only
upon the books of the Company which it shall cause to be maintained for such
purpose.  The Company may treat the registered Holder of this Warrant as he or
it appears on the Company's books at any time as the Holder for all purposes.
All warrants issued upon the transfer or assignment of this Warrant will be
dated the same date as this Warrant, and all rights of the holder thereof shall
be identical to those of the Holder.


     6.   Loss, Etc., of Warrant.  Upon receipt of evidence satisfactory to the
          ----------------------
Company of the loss, theft, destruction or mutilation of this Warrant, and of
indemnity reasonably satisfactory to the Company, if lost, stolen or destroyed,
and upon surrender and cancellation of this Warrant, if mutilated, the Company
shall execute and deliver to the Holder a new Warrant of like date, tenor and
denomination.


     7.   Warrant Holder Not Shareholder.  Except as otherwise provided herein,
          ------------------------------
this Warrant does not confer upon the Holder any right to vote or to consent to
or receive notice as a stockholder of the Company, as such, in respect of any
matters whatsoever, or any other rights or liabilities as a stockholder, prior
to the exercise hereof.


     8.   Communication.  No notice or other communication under this Warrant
          -------------
shall be effective unless in writing; such notice shall be deemed given on the
same day if sent by hand;

                                       4
<PAGE>

after three (3) business days (Saturdays, Sundays and a bank or other public
holidays excluded) if sent by certified mail, return receipt requested; after
one day if sent by reputable overnight courier service; and if sent by telex
upon receipt of the answer back signal; or if by facsimile upon receipt of the
confirmation slip showing completion of the transmission, addressed as follows:

          (a)    The Company at 137 Newbury Street, 8/th/ Floor, Boston, MA
02116 or other address as the Company has designated in writing to the Holder,
or

          (b)    the Holder [INSERT ADDRESS] or other such address as the Holder
has designated in writing to the Company.


     9.   Headings. The headings of this Warrant have been inserted as a matter
          --------
of convenience and shall not affect the construction hereof.

     10.  Applicable Law.  This Warrant shall be governed by and construed in
          --------------
accordance with the law of the State of Delaware without giving effect to the
principles of conflicts of law thereof.

IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by its
President and attested to by its Secretary.


                                         BOSTON LIFE SCIENCES, INC.



                                         By:_______________________
                                             President


ATTEST:


_______________________
   Secretary

                                       5
<PAGE>

                                  SUBSCRIPTION
                                  ------------

          The undersigned, _______________________, pursuant to the provisions
of the foregoing Warrant, hereby agrees to Subscribe for and purchase
________________ shares of the Common Stock, par value $.01 per share, of Boston
Life Sciences, Inc. covered by said Warrant, and makes payment therefor in full
at the price per share provided by said Warrant.

Dated:__________________                     Signature:_________________________

                                             Address:___________________________

                                                     ___________________________



                                  ASSIGNMENT
                                  ----------

          FOR VALUE RECEIVED______________ hereby sells, assigns and transfers
unto ______________________the foregoing Warrant and all right evidenced
thereby, and does irrevocably constitute and appoint __________________________,
attorney, to transfer said Warrant on the books of Boston Life Sciences, Inc.

Dated:__________________                     Signature:_________________________

                                             Address:___________________________

                                                     ___________________________



                              PARTIAL ASSIGNMENT
                              ------------------

          FOR VALUE RECEIVED __________________ hereby assigns and transfers
unto ________________________ the right to purchase ___________ shares of the
Common Stock, par value $.01 per share, of Boston Life Sciences, Inc. covered by
the foregoing Warrant, and a proportionate part of said Warrant and the rights
evidenced thereby, and does irrevocably constitute  and appoint
____________________________, attorney, to transfer that part of said Warrant on
the books of Boston Life Sciences, Inc.

Dated:__________________                     Signature:_________________________

                                             Address:___________________________

                                                     ___________________________

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