At fertilization, the binding of sperm to the zona pellucida induces the acrosome reaction releasing lytic enzymes that facilitate the passage of the sperm through the zona. The sperm then fuses with the egg's plasm membrane and enters the egg's cytoplasm. The zona pellucida, an extracellular matrix composed of three or four glycoproteins, mediates the species-specific binding of sperm to egg. Although homologous proteins are present in mouse and humans, human sperm will not bind to the mouse zona pellucida and when the zona pellucida is experimentally removed, human sperm will not bind or fuse to the mouse egg's plasm membrane. We have created transgenic mouse lines that express one or more human zona proteins. We wish to determine if human sperm will bind to these chimeric zonae and if this binding will induce the human sperm acrosome reaction. Sperm and devitalized eggs will be collected by collaborating institution(s) under protocols and consent forms approved by that institution's IRB. Alternatively, anonymous fertile human sperm will be obtained from a commercial sperm bank using a donor consent agreement. Only procedures already being performed on subjects for diagnostic or treatment purposes will be used.