The incorporation and fusion of capacitated, but not uncapacitated, spermatozoa with zona-free eggs in vitro have been reported for the hamster (Yanagimachi & Noda, 1970), rat (Niwa & Chang, 1975) and mouse (Wolf, Inoue & Stark, 1976). A requirement of sperm capacitation for penetration of zona-free hamster eggs by spermatozoa of the guinea-pig (Yanagimachi, 1972), mouse (Hanada & Chang, 1972), rat (Hanada & Chang, 1976) and man (Yanagimachi, Yanagimachi & Rogers, 1976) has also been reported. It has been suggested therefore that zona-free hamster eggs, and possibly the eggs of other laboratory animals, may be used as a model to evaluate the fertilizing capacity of the spermatozoa of different species (Yanagimachi, 1972; Yanagimachi et al., 1976). The present investigation was of the ability of boar spermatozoa to penetrate zona-free hamster eggs.