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Rabbit sperm acrosomes have a denuding effect on the zona pellucida, as Srivastava, Adams & Hartree (1965a, b) have demonstrated. This finding suggested that penetration of a spermatozoon through the zona pellucida is mediated, at least in part, by the presence of hydrolytic enzymes. Recently, Stambaugh & Buckley (1969) found hyaluronidase and a 'trypsin-like' enzyme in acrosomal extracts of rabbit spermatozoa; apparently the proteolytic enzyme is mainly responsible for the ability to penetrate the zona pellucida in this species. The localization ofproteolytic activity in intact sperm cells was accomplished by Gaddum & Blandau (1970), using an india-ink gelatin substrate method. The technique promised to be simple enough to allow further studies and even semiquantitative estimations of protease activity; however, in trying to repeat the method, we found that it gave inconstant results and obscured a great deal of the cytological details of the cells under study, nor was it possible to obtain clear-cut 'digestion halos' around the acrosomes, as illustrated in Gaddum & Blandau's original publication. We therefore attempted to develop a method that could clearly show proteolytic activity against gelatin, without significantly obscuring cytological detail. Human ejaculates and rat epididymal spermatozoa were used. The sperma¬ tozoa were washed twice in Hanks' solution with sodium bicarbonate 0-35 g/1 at a final pH of 7-4. The gelatin solution (5%) was prepared in a Veronalacetate buffer at a different pH ranging from 7-4 to 9. Washed spermatozoa were spread evenly on a glass slide; after 2 min at room temperature, smears were immersed once in the gelatin solution kept at about 28 to 30° C; the slides were set in a vertical position for a few minutes to obtain a thin gelatin film. After blotting the bottom edge, the slides were allowed to gel in a horizontal position in a humid chamber at room temperature from 12 to 48 hr. After incubation,

CYTOCHEMICAL DEMONSTRATION OF PROTEOLYTIC ACTIVITY OF HUMAN AND RAT SPERMATOZOA