Morphologically mature polymorphonuclear neutrophils (PMN) isolated from the peripheral blood of 30 patients were examined. These cells manifested the diminution of phagocytosis, random migration, chemotaxis, thermotaxis and bactericidal capacity. The surface charge of circulating PMN and the percent representation of cells with receptor for Fc IgG were significantly decreased. Stimulated as well as unstimulated oxygen consumption, production of O2– and H2O2 were significantly decreased, whereas the hexose monophosphate shunt activity was similar to that noticed in controls. The aggregation of leukemic PMN was practically normal, however their adherence was significantly higher. The possible mechanisms responsible for the observed PMN defects are discussed.