CPC10997 was found to be effective in vitro as an antiandrogen without effects on either the estrogen or the progesterone receptors in carcinomas of the breast, ovary and prostate as well as in malignant melanomas. Using the clonogenic assay of Salmon et al. [Cancer Treat. Rep. 65: 1-12, 1981], CPC10997 was found to be more effective against carcinomas of the breast, the kidney, the ovary and the prostate than conventional antineoplastic agents in the majority of tumors tested. In view of the very favorable toxicology profiles and in vitro efficacy, further trials using CPC10997 as an antineoplastic agent are indicated.