In this study, liver microsome-mediated activation of diphenyl (DP), diphenylmethane (DPM) and 2,2-diphenylpropane (DPP) to estrogens was demonstrated. These three compounds were negative in estrogen reporter assay using estrogen-responsive human breast cancer cell line MCF-7. However, they exhibited estrogenic activity after incubation with liver microsomes of 3-methylcholanthrene-treated rats in the cases of DP and DPM, or of phenobarbital-treated rats in the cases of DP and DPP, in the presence of NADPH. When these compounds were incubated with liver microsomes in the presence of NADPH, monohydroxyl and dihydroxyl derivatives were formed. These hydroxylated metabolites, 4-hydroxydiphenyl, 3-hydroxydiphenyl, 2-hydroxydiphenyl, 4-hydroxydiphenylmethane, 2-(4-hydroxyphenyl)-2-phenylpropane (4-OH-DPP), 4,4′-dihydroxydiphenyl, 4,4′-dihydroxydiphenylmethane and 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), all exhibited estrogenic activity in MCF-7 cells. Binding assay of these hydroxylated compounds with rat uterus estrogen receptor was also positive. These results suggest that the estrogenic activities of DP, DPM and DPP were due to the formation of hydroxylated metabolites by the liver cytochrome P450 system.