Human spermatozoa activated human polymorphonuclear leukocytes (PMN) in the presence of serum. Activation of PMN was studied by measuring the emission of chemiluminescence by the PMN. The amount of chemiluminescence emitted depended on the number of spermatozoa and the serum concentration, and the presence of antibody or complement. Spermatozoa were able to activate PMN in the presence of heat-inactivated serum (only antibody, no complement), serum obtained from a patient with agammaglobulinemia (without antibodies and only complement as opsonin), and in MgEGTA agammaglobulinemic serum (only the alternative pathway of complement intact). In the presence of heat-inactivated agammaglobulinemic serum no significant chemiluminescence was observed. It was concluded that spermatozoa activate the alternative pathway of complement. Dead spermatozoa were more able to activate PMN than viable spermatozoa.