Abstract. During the course of a study of glycoprotein processing mannosidases in the rat epididymis, we have made an intriguing discovery regarding the presence of a novel ot-o-mannosidase on the rat sperm plasma membranes. Unlike the sperm acrosomal "acid" mannosidase which has a pH optimum of 4.4, the newly discovered Ot-D-mannosidase has a pH optimum of 6.2, and 6.5 when assayed in sperm plasma membranes and intact spermatozoa, respectively. In addition, the two enzymes show different substrate specificity. The acrosomal Ot-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl a-Dmannopyranoside, whereas the sperm plasma membrane O~-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane Ot-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver.The newly discovered a-D-mannosidase appears to be an intrinsic plasma membrane component, since washing of the purified membranes with buffered 0.4 M NaCI did not release the enzyme in soluble form. The enzyme requires nonionic detergent (Triton X-100) for complete solubilization. The enzyme is activated by Co 2+ and Mn 2÷. However, Cu 2÷ and Zn 2÷ are potent inhibitors of the sperm plasma membrane Ot-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-c~-D-mannoside, methyl-ot-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface a-D-mannosidase. The physiological role of the newly discovered enzyme is not yet known. Several published reports in three species, including the rat, suggest that the sperm surface c~-o-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.I X is generally accepted that one step in the fertilization process requires interaction between complementary molecules present on the surface of the spermatozoon and the zona pellucida. The chemical nature of these complementary recognition sites is poorly understood, although there is growing evidence that carbohydrate moieties on surface membrane glycoconjugates are involved in these interactions (1, 34).In mammals, several sperm proteins have been suggested to bind to zona pellucida (28). Some of these macromolecules have enzymatic activity and are thought to form a stable enzyme-substrate complex by binding to the oligosaccharide units present on the zona pellucida glycoproteins. In mice, galactosyltransferase present on the head region of the spermatozoa mediates sperm-egg binding by interacting with its substrate on the zona pellucida (23). There is also evidence that trypsin-like protease present on spermatozoa of mouse (4, 31) initiates sperm-egg binding. Mouse spermatozoa also contain sialyltransferase (13) and fucosyltransferase (30). The latter enzyme has been suggested to be involved in some aspect of sperm-egg recognition (Apter, E M., ...