The timing of sperm penetration through the zona pellucida, and fertilization of human eggs had received little attention in the literature. Edwards et al. (1969) inseminated eggs matured in vitro with ejaculated spermatozoa which had been washed and found that spermatozoa did not penetrate the zona pellucida earlier than 7 h after insemination. A similar time interval between insemination and zona penetration by spermatozoa has been reported by others (Bavister et al., 1969; Overstreet and Hembree, 1976). This time interval has been considered to represent the time required for sperm capacitation, acrosome reaction and passage of the reacted spermatozoa through the zona pellucida (Austin, 1969; Austin et al., 1973). We report here that under our experimental conditions, sperm penetration into human eggs can occur much faster than previously reported.