An inhibitory effect of TZP-4238, a newly synthesized antiandrogen, on the growth of Dunning R3327 rat prostatic tumor was studied and compared with that of chlormadinone acetate (CMA). TZP-4238 markedly suppressed the growth of R3327 tumor. The inhibitory effect of TZP-4238 on the tumor was more potent than that on the prostate. While the inhibitory effect of TZP-4238 on the weight of the ventral prostate was about 6 times as great as that of CMA, the suppressive effect of TZP-4238 on tumor weight was about 40times as great as that of CMA. Serum levels of testosterone and dihydrotestosterone showed no obvious changes after the administration of either TZP-4238 or CMA. The inhibitory effect of TZP-4238 on the growth of R3327 tumor indicated the application of the compound to human prostatic cancer.