Individually different growth responses of 10 cell lines newly derived from metastasizing mammary carcinomas were determined by cell counts in experimental incubations with the steroid hormones 17 beta-estradiol, progesterone, testosterone, hydrocortisone (cortisol), the antiestrogenic compound tamoxifen, or prolactin. Of 7 cell lines derived from ductal carcinomas, 5 were stimulated by prolactin. The growth of 4 of 7 cell lines established from the tumors of postmenopausal or ovariectomized patients was enhanced by doses of testosterone, which are in the range of the physiologic serum level. The proliferation of 5 cell lines was promoted by hydrocortisone in the physiologic concentration of 100 nM, supporting the notion that concentrations of testosterone or hydrocortisone normally present in body fluids may facilitate the in vivo growth of breast cancer. The in vitro growth of cells derived from tumors after relapse under treatment with medroxyprogesterone acetate or tamoxifen was markedly enhanced by progesterone or tamoxifen (CAS: 10540-29-1) in concentrations corresponding to therapeutical serum levels and in accordance with in vivo resistance to the endocrine therapy applied before cell sampling. The results of this suggest the occurrence of positive endocrine selection mechanisms operating in vivo on human mammary tumor cell populations.