Members of the flavonoid class of phytochemicals have previously been demonstrated to possess estrogenic activity in a number of hormonally responsive systems. We have performed the present study to characterize the estrogenic and antiestrogenic activity of flavonoids in the estrogen receptor (ER)-positive MCF-7 human breast cancer cell line. Using an ER-dependent reporter gene assay and an ER competition binding assay, we have identified phytochemicals possessing estrogenic and antiestrogenic activities, which appeared to correlate directly with their capacity to displace [3H]estradiol from ER. Several flavonoids, including kaempferide, apigenin, and flavone, were distinct, in that their antiestrogenic activity did not appear to correlate with binding to ER, and therefore their suppression of estrogen-mediated gene transactivation and proliferation may occur independent of direct antagonism of the receptor. Further examination in HEK-293 cells transfected with ERalpha or ERbeta demonstrated potent antagonism with kaempferide and apigenin, while flavone was weakly antagonistic only toward ERP. These results suggest that the receptor binding-independent antiestrogenic chemicals may function through alternate signaling pathways as indirect ER modulators in a receptor- and cell type-specific manner. We conclude that antiestrogenic activities of flavonoid phytochemicals may occur through ER binding-dependent and -independent mechanisms and that the binding-independent antiestrogen activity of certain flavonoids is biologically significant in regulation of breast cancer cell proliferation.