The zona pellucida is an extracellular glycoprotein coat that surrounds the oocyte and, in many mammals, provides the first barrier sperm must penetrate for fertilization to occur. The molecular interactions between the sperm and zona pellucida appear to be relatively species-specific (1 ,2 ) and have been studied in most detail in the mouse. The mouse zona pellucida is composed of three glycoproteins termed ZP1, ZP2, and ZP3 in order of decreasing molecular weight (3 ).