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                                                                    Exhibit 5.1



                                    July 6, 2001

AVANT Immunotherapeutics, Inc.
119 Fourth Avenue
Needham, Massachusetts 02494
Attn: Dr. Una S. Ryan

         Re:      Legality of Securities to be Registered under
                  Registration Statement on Form S-3
                  -----------------------------------------

Ladies and Gentlemen:

         This opinion is delivered in connection with the registration statement
on Form S-3 (the "Registration Statement") pursuant to the Securities Act of
1933, as amended (the "Securities Act"), relating the sale from time to time of
10,000,000 shares of common stock, par value $.001 per share ("Common Stock"),
1,000,000 warrants to purchase shares of Common Stock ("Warrants") and the
rights (the "Rights") to purchase shares of Series C-1 Junior Participating
Cumulative Preferred Stock, which are attached to all shares of Common Stock
(collectively, the "Securities"), of AVANT Immunotherapeutics, Inc., a Delaware
corporation (the "Company"). The Registration Statement provides that the
Securities may be offered separately or together, in separate series, in
amounts, at prices and on terms to be set forth on one or more prospectus
supplements (each a "Prospectus Supplement") to the Prospectus contained in the
Registration Statement.

         In connection with rendering this opinion, we have examined the
Certificate of Incorporation and the Bylaws of the Company, each as amended to
date; such records of the corporate proceedings of the Company as we have deemed
material; a registration statement on Form S-3 under the Securities Act relating
to the Securities, and the prospectus contained therein; and such other
certificates, receipts, records and documents as we considered necessary for the
purposes of this opinion. We express no herein opinion concerning the laws of
any jurisdiction other than the laws of the United States of America and The
Commonwealth of Massachusetts and the Delaware General Corporation Law.

         Based upon the foregoing, we are of the opinion that (i) when the
Securities are specifically authorized for issuance by the Company's Board of
Directors or an authorized committee thereof (the "Authorizing Resolution"),
(ii) upon receipt by the Company of the full consideration therefor as provided
in the Authorizing Resolution and (iii) upon the issuance of the Securities as
described in the Registration Statement and a Prospectus Supplement that is
consistent with the Authorizing Resolution, the Securities will be legally
issued, fully paid and nonassessable.

         The foregoing assumes that all requisite steps will be taken to comply
with the requirements of the Securities Act and applicable requirements of state
laws regulating the offer and sale of securities.




GOODWIN PROCTER


         We hereby consent to being named as counsel to the Company in the
Registration Statement, to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to our firm under the caption "Legal
Matters" in the Prospectus.


                                                      Very truly yours,

                                                      /s/  GOODWIN PROCTER LLP

                                                      GOODWIN PROCTER LLP