The biological response on proliferation and progesterone receptor (PR) of the anti-estrogen ICI 164,384 [N-n-butyl-N-methyl-11-(3,17 beta-dihydroxyestra-1,3,5(10-trien-7 alpha-yl))-undecanamide] was studied in different mammary cancer cell lines. In the hormone-dependent cancer cell lines (MCF-7 and T-47D) this anti-estrogen significantly decreased cell proliferation, but to reduce 50% of the growth in the MCF-7 cells a very low concentration (10(-9) M) is necessary. Similar effects in the T-47D cell are obtained with a dose of 100-1000 times (10(-6)-10(-7) M). The stimulatory effect in cell proliferation induced by estradiol is also inhibited by ICI 164,384 in both cell lines. This anti-estrogen has no effect on proliferation in the anti-estrogen resistant cell line LY-2, or in the hormone-independent cell line MDA-MB-436. Studies on thymidine incorporation correlate with the effect on cell proliferation. ICI 164,384 also blocks the stimulatory effect on progesterone receptor provoked by estradiol in MCF-7 cells and in T-47D cells which contain high concentration levels of progesterone receptor ICI 164,384 significantly decreases the PR concentrations in both the non-treated and estradiol-treated cells. It is concluded that ICI 164,384 is a full antagonist in the hormone-dependent breast cancer cells, but it has no effect in the anti-estrogen-resistant or in hormone-independent cell lines.