In the present study the microbicidal activities of granulocytes and monocytes from AIDS patients (CDC group IV) were assessed and compared with those of healthy controls. The phagocytosis and intracellular killing of Staphylococcus aureus by patient and control cells were measured using a method in which the rate of intracellular killing can be assessed independently of the rate of phagocytosis. Both granulocytes and monocytes of AIDS patients showed a decreased phagocytosis of S. aureus in comparison to phagocytes of healthy individuals. The rates of intracellular killing of S. aureus by granulocytes and monocytes did not differ significantly between these patients with late-stage HIV infection and controls.