A.R.C. Unit of Reproductive Physiology and Biochemistry, 307 Huntingdon Road, Cambridge CB3 0JQ (Received 21st November 1974) Using histochemical techniques Mancini et al. (1964) established that hyaluronidase, a lysosomal enzyme, is localized in the acrosomes of bull spermatozoa. Acid phosphatase, the customary histochemical marker for lysosomes, is also concentrated mainly in the acrosomal regions of intact spermatozoa of several animal species (see papers quoted by Allison & Hartree, 1970). The finding of hyaluronidase in a suspension of ram sperm acrosomes (Srivastava, Adams & Hartree, 1965), and later of a number of other typical lysosomal hydrolases, including acid phosphatase (Allison & Hartree, 1970), led the latter authors to suggest that the acrosome is a specialized lysosome that has evolved to facilitate, by enzyme action, the sequential steps of penetration of the spermatozoon through the egg investments. Additional evidence for the lysosomal nature of the acrosome is provided by the analogous histochemical