We have examined the behavior of guinea pig spermatozoa towards the guinea pig zona pellucida in vitro. Experiments were designed to determine whether the state of capacitation or the acrosome reaction determined the ability of the spermatozoa to attach to the zona. Freshly collected cauda epididymal spermatozoa are acrosome-intact (AI) and, by definition, uncapacitated. These cells could not attach (at least persistently) to the zona. We then sampled spermatozoa from the progressively capacitating population at various time intervals. At no point did AI spermatozoa from such samples have the ability to attach to the zona. On the other hand, acrosome-reacted (AR) spermatozoa that appeared in increasing number during this capacitation period rapidly and firmly adhered to the zona. Adhesion by AR spermatozoa prepared in several ways culminated in successful zona penetration and fertilization of the ovum. The data provide unequivocal evidence that AR guinea pig spermatozoa attach to zonae and fertilize the ova. We find no evidence that AI spermatozoa firmly attach or bind to the zona, regardless of their state of capacitation.