Exhibit 99.4

                               SECOND ADDENDUM TO
                          REGISTRATION RIGHTS AGREEMENT

     This SECOND ADDENDUM TO REGISTRATION RIGHTS AGREEMENT (the "Addendum") is
made and entered into as of March 12, 2003, by and among Boston Life Sciences,
Inc., a Delaware corporation (the "Company") and each of the purchasers set
forth on the execution page hereof (the "Purchasers"). This Addendum supplements
that certain Registration Rights Agreement, dated as of July 25, 2002, by and
among the Company and the Initial Holders (as defined therein), as supplemented
by the First Addendum thereto dated as of November 22, 2002 (the "Registration
Rights Agreement"). Capitalized terms not defined herein shall have the meanings
given to them in the Registration Rights Agreement.

     WHEREAS, as of the date hereof, the Purchasers are acquiring an aggregate
of 10,000,000 shares of the Company's common stock (the "Shares") pursuant to
that certain Common Stock Purchase Agreement dated as of March 4, 2003 by and
among the Company and the Purchasers (the "Stock Purchase Agreement");

     WHEREAS, the execution of this Addendum is a condition to the closing of
the transactions contemplated by the Stock Purchase Agreement; and

     WHEREAS, Section 10(b) of the Registration Rights Agreement permits the
Company, without the consent of any Holder, to extend the registration rights
provided thereunder to additional persons who become holders of the Company's
equity securities;

     NOW, THEREFORE, in consideration of the premises and mutual covenants and
agreements set forth herein and other good and valuable consideration the mutual
receipt and sufficiency of which is hereby acknowledged, the parties to this
Addendum hereby agree as follows:

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     1.   Joinder of New Parties. By execution of this Addendum, each of the
Purchasers not already party to the Registration Rights Agreement is hereby
joined as a party to the Registration Rights Agreement and shall have all of the
rights and obligations of a Holder thereunder. The Shares (and any securities of
the Company issued or issuable with respect to the Shares by way of conversion,
exchange, dividend or stock split or combination) shall constitute Registrable
Shares under the Registration Rights Agreement (subject to the limitations set
forth in the second sentence of the definition of Registrable Shares in Section
1 of the Registration Rights Agreement).

     2.   Entire Agreement; Counterparts. Except to the extent specifically
supplemented by this Addendum, the Registration Rights Agreement shall remain
unmodified, and the Registration Rights Agreement, as supplemented hereby, is
confirmed as being in full force and effect. This Addendum may be executed in
two or more counterparts, all of which shall be considered one and the same
agreement and shall become effective when counterparts have been signed by each
party and delivered to the other party.

     3.   Miscellaneous. This Addendum shall be governed by the provisions of
Sections 12(e), (f) and (g) of the Registration Rights Agreement to the same
extent as if set forth in their entirety herein.

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     IN WITNESS WHEREOF, the parties hereto have executed this Addendum as of
the day and year first above written.

                                          BOSTON LIFE SCIENCES, INC.


                                          By: /s/ Robert J. Rosenthal
                                             --------------------------
                                              Name:  Robert J. Rosenthal
                                              Title: President


                                          PURCHASERS:

                                          [PURCHASER SIGNATURE BLOCKS]

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