Acrosin is a promising target for contraceptive agents1.Based on the previous homology modeling of human acmsin2, the multiple copy simultaneous search (MCSS) methodology3 was used to explore the active site of human acrosin.The results showed that all of three subsites (P1, P2 and G, Fig.l) are polar zones, and P1 is crucial for inhibitors binding.In addition, the edge of P1 and G as well as the bottom of P2 were hydrophobie, in which several important residues form hydrogen-binding interactions and eleetrostatic interactions.The key residues for ligand binding confirmed by MCSS calculations were consistent with the 3D structure of human acrosin complex and the experiments of site-directed mutagenesis on the animals.On the basis of above results, six of acrosin inhibitors(Tab.1), were docked into the active site of the human acrosin to elucidate their binding mode and the critical residues involved in binding.Our studies provide a basis for the rational design of novel acrosin inhibitors and the discovery of novel contraceptive drugs with high potency.