Human neutrophils activated by phorbol myristate acetate were cytotoxic to ox erythrocytes, as determined by the 51Cr release method. Maximal cytolysis was obtained with a phorbol myristate acetate concentration of 5 ng/ml and with an effector to target cell ratio of 1:4. An intact neutrophil metabolic burst and production of oxygen-derived-free radicals were essential for the cytotoxic event, since neutrophils from patients with chronic granulomatous disease failed to exhibit any ox erythrocyte lysis. The target cell destruction was completely prevented by catalase, was unaffected by superoxide dismutase, and was reduced approximately to one-third by azide and cyanide. These data suggest that, under our experimental conditions, the ox erythrocyte killing by phorbol myristate acetate-activated neutrophils mainly depends on myeloperoxidase and hydrogen peroxide.