Department of Anatomy and Reproductive Biology, University of Hawaii School of Medicine, Honolulu, Hawaii 96822, U.S.A. (Received 10th December 1974) Hyaluronidase which is localized in the sperm acrosome (Mancini, Alonso, Barquet, Alvarez & Nemirovsky, 1964) is released by ageing of the spermatozoa (Mann, 1964), freeze-thawing (Ackerman, 1970) and treatment of spermatozoa with a variety of reagents such as digitonin (Austin, 1960), Hyamine 2389 (Hartree & Srivastava, 1965) and MgCl2 (Srivastava, 1973). These conditions seem to destroy the integrity of the plasma and acrosomal membranes (Srivastava, Munnell, Yang & Foley, 1974). The release of hyaluronidase from living spermatozoa is believed to occur through a process called the acrosome reaction. This reaction, which involves multiple fusions between the outer acrosomal membrane and overlying sperm plasma membrane (Barros, Bedford, Franklin & Austin, 1967), occurs only after the spermatozoa attain a `capacitated' state (Bedford, 1970). In fertilization, the functional significance of the acrosome