Its acute leukemia, phagocytosis of Brucella organisms by mature neutrophils was not impaired. Phagocytosis of these organisms was not altered by antimetabolite therapy or during periods of bacterial infection. The absolute mature neutrophil count when infection developed was found to vary from patient to patient. The onset of bacterial infection in patients with acute leukemia was preceded by dynamic decreases in the number of mature neutrophils in the peripheral blood. Seventeen of eighteen patients had lower mature neutrophil counts immediately prior to the onset of infection as compared to the period free of infection. The median decrease was 54 per cent. As infection did not always follow decreases in mature neutrophils it is suggested that other factors also play a role in the development of bacterial infections in patients with acute leukemia.