A number of nonsteroidal estrogens, which are common naturally occurring substances in human foods, were examined for competitive binding to estrogen receptor proteins. These compounds bound competitively to estrogen receptor proteins in rat uterine cytosol, in tissue from 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumors, and in human mammary tumor tissue. The relative affinity of these estrogens for rat uterine cytosol receptors paralleled closely the affinities reported for other receptors. Oral administration of coumestrol did not appear to support the growth of DMBA-induced rat mammary tumors, nor did coumestrol act as an antiestrogen when administered orally in combination with 17 beta-estradiol. Coumestrol administered sc might, however, be able to support the growth of these tumors.