Leukocytes were obtained from the blood of healthy individuals by a method which did not expose the cells to anticoagulants, wettable surfaces, or excessive gravitational force. Blood was drawn into a Fenwal Blood Pack and the erythrocytes were sedimented with dextran. Leukocytes were coned, by centrifuging the supernatant plasma for 15 min. at 50 X G, and resuspending the cells in buffered saline. O2 consumption was measured by Warburg"s direct method. Carbohydrate utilization and lactaie formation (aerobic glycolysis) were detd. by chemical analyses. The effects of certain sugars, metabolic intermediates, and inhibitors on these processes were studied. Increasing the concn. of glucose from low levels to physiologic range increased the lactate production proportionately to the amt. of glucose added. In the presence of added glucose, O2 consumption diminished. Fructose did not increase lactate formation comparably to glucose and was without effect on respiration. Both fructose and glucose reduced glycogenolysis but the effect of the latter was greater. Low O2 tension did not alter the high rate of aerobic glycolysis of mature leukocytes. The effect of metabolic intermediates and inhibitors indicated the existence of Krebs" cycle activity in the human polymorphonuclear leukocyte.