Using zona-free mouse eggs treated with mannosidase, the role of mannose on the surface of eggs in fertilization was examined. The fertilization rates of eggs treated with mannosidase at concentrations of 0.05, 0.1, 0.5 or 1.0 units/100 µl were 47.8, 40.4, 28.2 and 18.8%, respectively, which were all significantly lower than the 69.2% of control eggs with no mannosidase treatment. The binding of Canavalia ensiformis agglutinin (Con A), a lectin with an affinity to mannose, was strong on the surface of all control eggs not treated with mannosidase. However, the rate of eggs showing strong Con A-binding was significantly decreased among those treated with mannosidase at a concentration of 0.1 units/100 µl (44.0%). These findings suggest that mannose on the surface of the cytoplasmic membrane, in addition to zona pellucida, of eggs has a possible role in sperm recognition.