The antiproliferative and cytotoxic effects of 4-hydroxytamoxifen, an antiestrogen with a high affinity for the estrogen receptor, and of 17 beta-hydroxy-11 beta-(4-methylaminophenyl)-17-(1-propynyl)estra-4,9-dien-3- one-6-7 (RU486), an antiprogestin with a high affinity for the progestin receptor, have been studied on human breast cancer cell lines in culture. The number of dead cells was evaluated by several techniques (trypan blue stain exclusion, DNA cleavage, lactic dehydrogenase activity, morphological changes, and cloning efficiency in soft agar) and found to be increased both by the antiestrogen and the antiprogestin at concentrations correlating with the affinities for their respective receptors. This cytotoxic effect was prevented by the occupation of the respective receptors with estrogen and progestin and was not found in the estrogen receptor- and progestin receptor-negative MDA MB 231 and BT20 cell lines. The contrast between the ultrastructural modifications of chromatin and the integrity of mitochondria suggested that the antihormone-induced cell death was by apoptosis. We conclude that in addition to the receptor-mediated cytostatic activity and the nonspecific cytotoxic activity, antiestrogens trigger a third type of effect that we designate as "receptor-mediated cytotoxic." Similar conclusions can be drawn for the antiprogestin RU486, indicating moreover that the antihormone and antiproliferative activities of this drug are clearly dissociated. The mechanism of these receptor-mediated cytotoxic activities of antiestrogen and antiprogesterone is not known but does not seem to be explained entirely by the antihormone activity of these drugs.