Summary A mixture of 1% phosphotung-stic acid in 10% chromic acid selectively and differentially stains the plasma and acrosome membranes of ejaculated porcine sperm. Prior to staining, the cells or cell fractions are fixed with glutaraldehyde–osmium tetroxide, embedded in Epon, and, after sectioning, destained with periodic acid. Plasma membranes are stained intensely by the procedure while a fibrous component is revealed on the inner surface of acrosome membranes. Other sperm membranes do not show these staining characteristics. The specificity of the stain for plasma and acrosome membranes is retained in crude homogenates and purified cell fractions.