EXHIBIT 5.1

                           [ROPES & GRAY LETTERHEAD]

                                 August 23, 2000



Boston Life Sciences, Inc.
137 Newbury Street
Boston, Massachusetts 02116

         Re: Boston Life Sciences, Inc.

Ladies and Gentlemen:

         This opinion is furnished to you in connection with a registration
statement on Form S-3 (the "Registration Statement"), filed today with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
for the registration of 150,141 shares of the Common Stock, $.01 par value, of
Boston Life Sciences, Inc., a Delaware corporation (the "Company"), issuable
upon the exercise of certain warrants issued to the Series C stockholders (as
enumerated in the Registration Statement under the captions "Issuance of Common
Stock and Warrants to Selling Stockholders" and "Selling Stockholders"). The
foregoing Warrants are referred to collectively herein as the "Warrants," and
the Common Stock issuable upon exercise of the Warrants is referred to
collectively herein as the "Warrant Shares."

         The Company originally sold the Warrants to certain Series C
stockholders pursuant to the terms of an Offer Letter dated November 15, 1999
from the Company to the Series C stockholders. We understand that the Warrant
Shares are being registered to permit the secondary trading of such Common Stock
by the holders thereof from time to time after the effective date of the
Registration Statement.

         For purposes of this opinion, we have examined and relied upon such
documents, records, certificates and other instruments as we have deemed
necessary. We have examined one of the Warrants furnished by the Company to us,
to wit, Warrants No. BLSI-J-110, and have assumed, based upon a representation
of the Company, that the other Warrants are identical in all respects to the
Warrant we examined (other than the number of underlying Warrant Shares, the
name of the warrantholder and the warrant number).

         We express no opinion as to the applicability of, compliance with or
effect of Federal law or the law of any jurisdiction other than The Commonwealth
of Massachusetts and the corporate laws of the State of Delaware.

         Based on the foregoing, we are of the opinion that the Warrant Shares
have been duly authorized and when the Warrant Shares are issued out of the
Company's duly authorized Common Stock upon exercise of, and pursuant to the
provisions of, the Warrants and the Company has received the consideration
therefor in accordance with the terms of the Warrants, the Warrant Shares will
be validly issued, fully paid and non-assessable.

         We hereby consent to your filing this opinion as an exhibit to the
Registration Statement and to the use of our name therein and in the related
prospectus under the caption "Legal Matters".

         It is understood that this opinion is to be used only in connection
with the offer and sale of the Warrant Shares while the Registration Statement
is in effect.

                                                     Very truly yours,

                                                     /s/ Ropes & Gray
                                                     Ropes & Gray