A stable, tamoxifen-resistant subline, MCF-7/TAMR-1, of the human breast cancer cell line MCF-7 has been established in tissue culture after long-term treatment with 10(-6) M tamoxifen. The MCF-7/TAMR-1 cell line grows equally well in the presence and absence of tamoxifen, whereas the steroidal antiestrogens ICI 164,384 and ICI 182,780 exert profound inhibitory activity on cell proliferation, although higher concentrations are required to inhibit these cells compared to the parent cells. The MCF-7/TAMR-1 cells grown in tissue culture deviate from parent characteristics by the complete lack of expression of progesterone receptors even when grown with estradiol, by an altered tamoxifen regulation of M(r) 52,000 cathepsin D synthesis and secretion, and by lack of tamoxifen stimulation of an estradiol down-regulated M(r) 42,000 protein with presumed growth inhibitory function. MCF-7/TAMR-1 cells are estrogen receptor positive. The estrogen receptors have wild-type characteristics with respect to (a) binding of estradiol, tamoxifen, and ICI 164,384; (b) estrogen and antiestrogen regulation of the estradiol-regulated proteins pS2, M(r) 61,000 alpha 1-antitrypsin-like protein, M(r) 66,000 alpha 1-antichymotrypsin-like protein, and corresponding mRNAs; and (c) estrogen and antiestrogen regulation of a transiently transfected estrogen responsive reporter gene. We suggest that the lack of tamoxifen up-regulation of the M(r) 42,000 protein synthesis in MCF-7/TAMR-1 cells may at least partly explain the resistance to tamoxifen treatment. The sensitivity to the growth inhibitory activity of ICI 164,384 and ICI 182,780 may be ascribed to the maintenance of the pure antagonistic effect of these steroidal antiestrogens on MCF-7/TAMR-1 cells. Our results indicate that treatment with pure antiestrogens may be effective when patients become refractory to tamoxifen therapy.