The pre-penetration contact interactions between eggs and spermatozoa of the golden hamster were studied in vitro. The spermatozoa arrive at the surface of the egg and become loosely associated with the egg (`attachment'). `Binding', the species-specific tight association between gametes, occurs in two steps. The first (B1) occurs within 2 to 3 min after the spermatozoa and eggs are mixed and is rapidly terminated. By contrast, binding to the isolated zona pellucida is not terminated. Termination must therefore involve a factor originating from the vitellus or perivitelline space. Experiments with the synthetic trypsin inhibitor, p-aminobenzamidine (pABA), suggest that termination involves a protease, since B1 was enhanced when eggs were exposed to the inhibitor during the B1 phase. Following the termination of B1, in the normal interaction, the spermatozoa remain attached and apparently are conditioned through the action of the vitelline factor to bind a second time. This binding (B2) occurs about 30 min after B1 termination. The B2 process may involve the acrosomal proteinase of the sperm head, since B2 was inhibited by treating spermatozoa with pABA. In contrast to the B2 site, B1 and binding to isolated zonae pellucidae were not blocked by pABA treatment of spermatozoa. This suggests that the binding phases, B1 and B2, occur at different sperm sites.