The interactions of phytoestrogens with estrogen receptors were studied in the human breast cancer cell line, MCF-7. The compounds tested were coumestrol, genistein, and formononetin and the mycotoxins, zearalenone and its reduced derivative, zearalenol. All but formononetin compete for binding of [3H]-estradiol to unfilled cytoplasmic estrogen receptor or unfilled nuclear estrogen receptor sites. Relative binding affinities are zearalenol HMP (high melting point isomer) greater than zearalenol LMP (low melting point isomer) greater than zearalenone = coumestrol greater than genistein greater than formononetin. Dissociation constants estimated from competition curves show that binding affinities are high. In contrast to estradiol, phytoestrogens bind only weakly to sex steroid-binding globulin; they also do not bind to corticosteroid-binding globulin. These compounds translocate the cytoplasmic estrogen receptor and bind to unfilled nuclear estrogen receptors in whole cells. Bound nuclear receptors are then processed in a manner similar to estradiol in a step which rapidly decreases total cellular estrogen receptors. The phytoestrogens are also biologically active; they can markedly enhance tumor cell proliferation. In sum, phytoestrogens interact with the estrogen receptors of human breast cancer cells in culture and, therefore, may affect estrogen-mediated events in these cells.