The relative ability of the antiestrogen, tamoxifen, and its monohydroxy and desmethyl metabolites to displace tritiated estradiol from estrogen receptors isolated from human breast carcinoma was determined. 4-Hydroxytamoxifen binds to the estrogen receptor with affinity equal to estradiol, and with 25-50 times higher affinity than does tamoxifen. Desmethyltamoxifen binds to the estrogen receptor with less than 1 per cent of the affinity of tamoxifen. Blood level determination of tamoxifen species in breast cancer patients receiving the drug indicated that after 3 weeks of therapy, desmethyltamoxifen levels were 1.2-1.8 times greater than tamoxifen levels and 4-hydroxytamoxifen levels were 2-18 per cent of parent drug concentrations. Thus, because of its high affinity for the estrogen receptor, the 4-hydroxy metabolite may play a significant antiestrogenic role following tamoxifen therapy, despite its low relative abundance. Desmethyltamoxifen, on the other hand, may have only minor importance as an anti-estrogenic agent following tamoxifen treatment, due to its poor affinity for the estrogen receptor coupled with blood levels that are at most 75 per cent greater than tamoxifen levels at steady state.