Two classes of progestagens, e.g. pregnane [Org 2058, progesterone (PROG), R5020, medroxyprogesterone acetate (MPA)] and 19-nortestosterone derived progestagens [norethisterone (NE), levonorgestrel (LNG), 3-ketodesogestrel (KDG), gestodene (GES), Org 30659] were studied for their effect on cell growth of two human breast tumor T47D cell lines of different origin, i.e. from ATCC (A) and Sutherland (S) et al. [Sutherland et al., Cancer Res. 48 (1988) 5084-5091]. The effect of estradiol (E2) and progestagens alone as well as the combined effect of E2 (10(-10) M) and progestagens were investigated at several dose levels. Compared with E2-induced growth at 10(-10) M, pregnane and 19-nortestosterone derived progestagens at 10(-6) M alone did enhance cell growth in T47D-A cells up to 25 and 100% respectively, whereas in T47D-S cells they did not influence growth. All these progestagens at 10(-6) M did not affect E2-induced growth in T47D-A cells, whereas in T47D-S cells they completely reduced cell proliferation at doses between 10(-10) and 10(-8) M. The involvement of progestagen (PR) and estrogen (ER) receptors with respect to growth stimulation was studied by using specific antihormones. In T47D-A cells, the antiprogestagens RU 38486 and Org 31710 could not block progestagen-induced growth. Antiestrogens, like 4-hydroxytamoxifen and ICI 164,384, inhibited the 19-nortestosterone derivative-induced cell growth by approx. 50%. Remarkably, both antiprogestagens alone could also inhibit E2-induced growth in T47D-A cells by about 50%. In T47D-S cells, E2-induced cell growth was completely blocked by both antiprogestagens and antiestrogens. Both antiprogestagens in T47D-S cells were equipotent to 4-hydroxytamoxifen and 10-fold more potent than ICI 164,384. In conclusion pregnane and 19-nortestosterone-derived progestagens stimulated cell growth in T47D-A cells at high unphysiological concentrations, whereas they did not affect cell growth in T47D-S cells. The 19-nortestosterone derivative induced growth in T47D-A cells could partially be inhibited by antiestrogens. In T47D-A cells, E2-induced cell growth was not influenced by both classes of progestagens, whereas in T47D-S cells all tested progestagens, antiprogestagens, and antiestrogens inhibited E2-induced cell growth completely. These results with T47D cells as well as those obtained previously with MCF-7 cells show that subclones of cell lines may respond differently to various types of progestagens in the presence and absence of estrogens.