Human spermatozoa do not attach to the zona pellucida of other species because of their high specificity compared to that of other animals in recognizing the zona pellucida. This study describes the recognition mechanism of human spermatozoa through trypsin treatment of the ovum of human and other species. This research has revealed two findings: (1) The "lattice-like" structure, the surface of the zona pellucida of the hamster ovum, was lost and became a uniformly coarse and flat structure. More than 200 human spermatozoa attached to it. The number of attaching spermatozoa increased significantly along with the prolongation of the incubation period and in direct dependence on the spermatozoa concentration. (2) Human spermatozoa did not attach to the zona pellucida of mouse, rat, and human ovum. This result confirmed that human spermatozoa attach to hamster ovum if modified with trypsin.