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The zona pellucida (ZP) glycoproteins play a vital role in mammalian fertilization. In most species, the zona pellucida is composed of at least three biochemically distinct glycoproteins: ZP1, ZP2 and ZP3. In the mouse model, spermatozoa initially bind to ZP3, which triggers the acrosome reaction (Bleil and Wassarman, 1983). Subsequent to the acrosome reaction, ZP2 acts as the secondary receptor and helps in the maintenance of binding of acrosome-reacted spermatozoa to the egg (Bleil et al., 1988). ZP1 has been postulated to cross-link the ZP2–ZP3 heterodimer. ZP2 is proteolytically cleaved after fusion of the sperm membrane with the oolemma and this modification, together with presumed changes in ZP3, is postulated to play an important role in the post-fertilization block to polyspermy (Moller and Wassarman, 1989). This critical role during fertilization has made the ZP glycoproteins potential candidate antigens for immunocontraception. Antibodies generated against the ZP glycoproteins purified from a given species show variable degrees of immunological crossreactivity with the ZP glycoproteins from other species (Sacco et al., 1981). The immunological crossreactivity among ZP glycoproteins from different species is a result of a variable degree of amino acid sequence homology and this property has allowed the possibility of heterologous immunization. Initially, pig ZP glycoproteins were used because of the availability of a large number of pig ovaries; the immunological crossreactivity of antibodies generated in this way with zonae pellucidae from various species was demonstrated. Studies in several animal models have …