The comparative in vitro spermicidal effects of (+)-gossypol, (-)-gossypol and (+/-)-gossypol were evaluated on the spermatozoa of human, monkey, rabbit, mouse, rat and hamster. The spermicidal effects of gossypol isomers were also compared with those of gossypolone, which is a proposed major metabolite of gossypol. Gossypol isomers and gossypolone were all spermicidal. (+)- and (-)-Gossypol demonstrated spermicidal activities at the same concentration at which (+/-)-gossypol shows spermicidal effects on the spermatozoa of all species tested. However, gossypolone was less potent than the gossypol isomers. The spermicidal action of gossypol may be a nonspecific effect unrelated to the antifertility mechanism of orally administered gossypol, since (+)-gossypol which is not an effective male antifertility agent also showed the equivalent spermicidal effect to that of (+/-)-gossypol.