Studies from electronic microscopy disclosed the presence of an electrodense stranded and branch-like electrodense layer that extends toward extracellular space. Chemical composition of this glycoproteic layer showed that protein and total sugar content is similar (0.98 microgram/microgram protein). As for the total sugar content of this glycoproteic constituent, sialic acid accounts for 40%, hexosamines 27%, and fucose 30%. Electrophoresis characterization of this constituent showed the presence of 6 different motility bands. Risen levels of sialic acid indicate the contribution of sialic residue in the net charge of the sperm membrane, its role during capacitation, and its possible participation in the formation of binding bridges between sperm membrane and ovum.