The phagocytotic activity of monocytes from diabetic patients and healthy controls was studied. It was found that the number of phagocytizing cells from diabetic patients was significantly reduced in comparison with that from control individuals. However, the number of bacteria phagocytized per cell was similar in both groups. Plasma from healthy controls added to diabetic monocytes did not cause any significant change in their phagocytotic capacity. Addition of insulin to the plasma of diabetic patients failed to alter the number of phagocytizing diabetic monocytes. Similarly, addition of glucose to control plasma did not affect the number of control monocytes capable of phagocytosis. Protein synthesis was increased during phagocytosis in both control and diabetic cells. The importance of monocytes in the defense mechanism of the organism is discussed.