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                                                                     EXHIBIT 5.1

                                VENTURE LAW GROUP
                           a Professional Corporation
                               2800 Sand Hill Road
                              Menlo Park, CA 94025

   
                               December 14, 1998
    

Celtrix Pharmaceuticals, Inc.
3055 Patrick Henry Drive
Santa Clara, California 95054

     REGISTRATION STATEMENT ON FORM S-3

Ladies and Gentlemen:

   
     We have examined the Registration Statement on Form S-3 (as amended by
Amendment No. 1) to be filed by you with the Securities and Exchange Commission
on or about December 14, 1998 (the "Registration Statement") in connection with
the registration under the Securities Act of 1933, as amended, of a total of
10,075,000 shares of your Common Stock (the "Shares"), 4,000,000 of which Shares
were issued to certain individuals and entities (the "Stockholders") in a
private placement on November 20, 1998, and 6,075,000 of which Shares are
issuable upon the exercise of warrants (the "Warrants") issued to such
Stockholders in connection with such private placement. As your legal counsel,
we have examined the proceedings taken in connection with the sale of the Shares
and the Warrants to the Stockholders and are familiar with the proceedings
proposed to be taken by you, and the Stockholders in connection with the sale of
the Shares under the Registration Statement.
    

     It is our opinion that the Shares are legally and validly issued, fully
paid and nonassessable (or in the case of the Shares issuable upon exercise of
the Warrants, will be legally and validly issued, fully paid and nonassessable
upon exercise of the Warrants in accordance with their terms) and when resold in
the manner referred to in the Registration Statement, the Shares will be legally
and validly issued, fully paid and nonassessable.

     We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to the use of our name wherever it appears in the
Registration Statement and any amendments to it.

                                       Sincerely,

                                       VENTURE LAW GROUP
                                       A Professional Corporation


                                       /s/ Venture Law Group
                                       ---------------------------

ESR