Gossypol is a polyphenolic aldehyde occurring naturally in cottonseed that produces antisteroidogenic activity in vivo, has been extensively investigated as a male contraceptive agent, and has demonstrated anticancer activity. Gossypolone, the major metabolite of gossypol, also prossesses antisteroidogenic activity but has not been examined for its anticancer properties. The objectives of these investigations are to compare the effects of gossypolone with those of gossypol on cell proliferation of hormone-dependent and hormone-independent human breast carcinoma cells, i.e., MCF-7, MCF-7 Adr and MDA-MB-231 cells. Gossypol and gossypolone were examined at concentrations up to 10 microM, and cellular DNA synthesis was monitored by 3H-thymidine incorporation. Gossypol and gossypolone produced dose-dependent suppression of DNA synthesis in all of the human breast cell lines examined. Gossypol produced potent antiproliferative activity in MCF-7 cells at doses as low as 30 nM. Co-incubation of MCF-7 cells with gossypol (5 microM) and estradiol (10 nM) did not alter the effects of gossypl. Treatment of human breast cancer cells with 2.5 microM of gossypol resulted in alterations in cell shape and attachment to the surface of the culture dishes. At gossypol doses of 10 microM, pericytoplasmic globuation and cytoplasmic swelling were observed in the majority of breast cancer cells. These changes in cellular morphology indicate a loss of ability of the cells to maintain normal cell membrane permeability, resulting in subsequent disorganization and loss of cytoplasmic organelles. Gossypolone is less potent than gossypol in producing these effects in the human breast cancer cell lines, whereas it possesses equipotent antisteroidogenic and antireproductive activities with gossypol. These investigations suggest that gossypol and gossypol analogs may have therapeutic potential for human breast cancer.