Dietary estrogens are believed to exert their estrogenic or antiestrogenic (chemopreventive) action in estrogen responsive cells by interacting with the estrogen receptor (ER). The present study was undertaken to evaluate a direct role of ER in estrogenic or antiestrogenic activities of three dietary estrogens (coumestrol, genistein and zearalenone). HeLa cells were transiently co-transfected with an expression vector for ER and an estrogen-responsive reporter gene construct. Coumestrol, genistein, and zearalenone all increased the activity of the reporter gene, only in the presence of the ER, and the activation was blocked with the ER antagonist ICI 164,384, demonstrating an ER-specific, agonist response. In addition, in MCF-7 cells, coumestrol and zearalenone increased the expression of the estrogen-responsive pS2 gene. Coumestrol and genistein inhibited the purified estrogen-specific 17ß-hydroxysteroid oxidoreductase enzyme and the conversion of estrone to 17ß-estradiol in T-47D cells, which contain this enzyme. However, they did not inhibit the estrone-induced proliferation of T-47D cells. In conclusion, coumestrol, genistein, and zearalenone are all potent estrogens in vitro, and they act through ER mediated mechanism. Our findings give no evidence to support the idea that these compounds act as antiestrogens through competition for the binding sites of ER or by inhibition of the conversion of estrone to 17ß-estradiol in breast cancer cells, since this effect was nullified by their agonist action on cell proliferation. Therefore, their suggested chemopreventive action in estrogen-related cancers must be mediated through other mechanisms.