Freshly collected blood sera of the hamster and other species (rat, guinea pig, rabbit, bull, and human) were very toxic to hamster spermatozoa. When these sera were pretreated with heat (56°, 30–35 min), they became nontoxic and capable of inducing capacitation of hamster spermatozoa. Heat-treated (detoxified) hamster serum was most effective in capacitating the spermatozoa, and bovine serum was least effective. Among various commercially produced sera tested, gamma globulin-free human serum was the most potent in capacitating the spermatozoa. Efficient sperm capacitation was induced in media containing both dialyzable and albumin fractions of the sera. When one of these two fractions was absent from the medium, no capacitation or only very inefficient capacitation was induced.