The density and affinity of tamoxifen (TAM) binding sites in isolated plasma membrane fractions from human ovarian tissue and ovarian tumors was measured. TAM binding in both membrane preparations was specific, saturable, and had a high affinity (KD < 1 nM). The density of TAM binding sites in the tumors was at least three times higher than that in the normal ovaries, whereas the affinity of TAM binding sites in the tumors was lower than in normal ovaries. The relatively large number of TAM binding sites that are distinct from estrogen receptors in ovarian tumors suggest that TAM alone and in combination with other chemotherapy could be used with advantage in ovarian cancer.