Human spermatozoa are initially incapable of penetrating zona pellucida-free animal (hamster) ova but gain the ability to do so when incubated several hours in vitro. Experimental evidence suggests that this ability is associated with the completion of sperm capacitation and the acrosome reaction. Thus it appears that zona-free animal ova can be substituted for human ova in the preliminary assessment of the fertilizing capacity of human spermatozoa when human ova are not readily available.