EXHIBIT 32.1

                            CERTIFICATION PURSUANT TO
                            18 U. S. C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-0XLEY ACT OF 2002

In connection with the Quarterly Report of GTx, Inc. (the "Company") on Form
10-Q for the three months ended March 31, 2006, as filed with the Securities and
Exchange Commission on the date hereof (the "Report"), I, Mitchell S. Steiner,
Chief Executive Officer of the Company certify, pursuant to Rule 13a-14(b) or
Rule 15d-14(b) and 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of
the Sarbanes-Oxley Act of 2002, that:

1.   The Report fully complies with the  requirements  of Section 13(a) or 15(d)
     of the Securities Exchange Act of 1934; and

2.   The information contained in the Report fairly presents, in all material
     respects, the financial condition and results of operations of the Company.

Date: May 5, 2006

/s/ Mitchell S. Steiner
- ---------------------------------------
Mitchell S. Steiner, M.D., F.A.C.S.
Chief Executive Officer and
Vice-Chairman of the Board of Directors

This certification accompanies the Form 10-Q to which it relates, is not deemed
filed with the Securities and Exchange Commission and is not to be incorporated
by reference into any filing of the Company under the Securities Act of 1933, as
amended, or the Securities Exchange Act of 1934 (whether made before or after
the date of the Form 10-Q), irrespective of any general incorporation language
contained in such filing.