The studies reported here demonstrate that isolated hamster zonae pellucidae affect the binding between capacitated hamster spermatozoa and eggs included in the same drop. Mouse zonae pellucidae fail to affect binding between hamster gametes. These results suggest that binding, which occurs only between homologous spermatozoa and isolated zonae pellucidae causes a factor (S1 factor) to be released. The SI factor is diffusible and influences the two successive binding steps (B1 and B2) previously shown to occur between capacitated spermatozoa and eggs. The B2 step is delayed by the action of the SI factor during the early part of the interval between B1 and B2 suggesting that the effect may be mediated through Bl. It is proposed that the SI factor is also released during the first binding interaction (B1) between the spermatozoa and the egg which appears to be an analogous but terminated form of the binding to the isolated zona pellucida. The data are consistent with the possibility that the B1 step is terminated by the product of the interaction between the S1 factor and a factor from the vitellus or perivitelline space.