This article concerns a study on the endocytic functions of circulating monocytes from 12 patients with acute or chronic monocytic leukemia. The results show that phagocytosis and intracellular killing of Staphylococcus aureus are impaired in only two patients and that the opsonic activity of the serum of all patients is normal. With respect to the intracellular killing of ingested Staphylococcus aureus, an interesting phenomenon was found in that the cells of patients with monocytic leukemia proved to be in a state of activation, as shown by the finding that patients' monocytes with normal phagocytosis killed about 64% of the ingested bacteria in the absence of extracellular stimulation by serum factors. When extracellular serum was present, the mean killing index rose to only 69%. This is unlike the situation seen in monocytes from healthy donors, where no killing occurs in the absence of extracellular serum and extracellular stimulation by serum factors is mandatory for optimal intracellular killing.