Quantitative electron microscopy is used to provide new information regarding the purity and the amounts of plasma membrane originating from various regions of the surface of the boar spermatozoon when such membranes are prepared by nitrogen cavitation and sucrose density centrifugation. These data show that the plasma membranes originate primarily from the head of the spermatozoon; they also show that contamination of these plasma membranes by acrosomal and mitochondrial membranes is negligible. Immunolocalization, using plasma membrane vesicles and divalent IgG directed against plasma membrane antigens, shows that plasma membrane vesicles bind extensively to intact sperm plasma membranes but rarely bind to the outer acrosomal membrane. These new data, taken together with that which has been previously reported, indicate that high pressure gas cavitation provides a simple means for obtaining undamaged, highly purified sperm plasma membranes.