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                                                                     EXHIBIT 5.1
                          [PORTER & HEDGES LETTERHEAD]




                                  July 20, 2000


Securities and Exchange Commission
Judiciary Plaza
450 Fifth Street, N.W.
Washington, D.C. 20549


         Re:  TEXAS BIOTECHNOLOGY CORPORATION REGISTRATION STATEMENT ON FORM
              S-8; AMENDED AND RESTATED 1995 NON-EMPLOYEE DIRECTOR STOCK OPTION
              PLAN

Ladies and Gentlemen:

         We have acted as counsel to Texas Biotechnology Corporation, a Delaware
corporation ("Company"), in connection with the preparation for filing with the
Securities and Exchange Commission of a Registration Statement on Form S-8
("Registration Statement") under the Securities Act of 1933, as amended. The
Registration Statement relates to the registration of an additional 200,000
shares ("Shares") of the Company's common stock, par value $.005 per share
("Common Stock"), issuable pursuant to the Company's Amended and Restated 1995
Non-Employee Director Stock Option Plan, as amended (the "Director Plan").

         We have examined the Director Plan and such corporate records,
documents, instruments and certificates of the Company, and have reviewed such
questions of law as we have deemed necessary, relevant or appropriate to enable
us to render the opinion expressed herein. In such examination, we have assumed
without independent investigation the authenticity of all documents submitted to
us as originals, the genuineness of all signatures, the legal capacity of all
natural persons, and the conformity of any documents submitted to us as copies
to their respective originals. As to certain questions of fact material to this
opinion, we have relied without independent investigation upon statements or
certificates of public officials and officers of the Company.

         Based upon such examination and review, we are of the opinion that the
Shares have been duly and validly authorized and will, upon issuance and
delivery as contemplated by the Director Plan, be validly issued, fully paid and
nonassessable outstanding shares of Common Stock.

         This Firm consents to the filing of this opinion as an exhibit to the
Registration Statement.


                                                 Very truly yours,

                                                 /s/ Porter & Hedges, L.L.P.

                                                 PORTER & HEDGES, L.L.P.