Evidence is accumulating that estradiol metabolites may be involved in carcinogenesis as some metabolites exert proliferative and others anti-proliferative properties on human cancer cells. The present study is the first to investigate the effect of 14 endogenous estradiol metabolites on the proliferation of the human breast cancer cell line, MCF-7, in comparison with the effect of the parent substance 17beta-estradiol with special concern on high pharmacological concentrations. The steroids were tested in the range from 10(-8) to 10(-5) M on MCF-7 cells which were incubated for nine days. Estradiol and almost all A-ring metabolites displayed biphasic reactions on cell proliferation, i.e. stimulatory at low concentrations and inhibitory at the highest concentration, 10(-5) M. The D-ring metabolites did not show such clear biphasic patterns, in most of them the stimulatory effect prevailed at the highest dosage used. The strongest inhibitory effect was seen for the A-ring metabolite 2-methoxyestradiol at the concentrations of 10(-6) and 10(-5) M and the strongest stimulatory effect was noted for the D-ring metabolite estriol at the same concentrations. The results indicate that some A-ring metabolites might be suitable for breast cancer treatment when used in high dosages. This is of special interest, since many of these metabolites have very weak estrogenic activity.