Female sheep were injected with highly purified and partially purified preparations of ram sperm acrosin and hyaluronidase. The fertility and immune response of the sheep were monitored. Fertility was not significantly reduced in any single group, though a positive correlation was observed between high antibody titres against acrosin and reduced fertility. Studies on the direct action of sera from the ewes on ejaculated ram spermatozoa did not show any evidence of sperm agglutination or immobilization. Similar studies with denuded spermatozoa (detergent induced 'acrosome reaction') sometimes resulted in agglutination and enzyme inhibition was also seen; there was no correlation between any of these parameters and pregnancy.