It has been assumed that patients with diabetes mellitus (DM), who are usually susceptible to bacterial infections, have a depression of the antibacterial defense mechanism. This study was undertaken to substantiate the hypothesis that phagocytosis of the reticuloendothelial system (RES) is depressed in diabetic patients. It is believed that intravenously injected colloidal suspensions whose particles are unable to pass through the capillary barrier will be phagocytized mainly by the reticuloendothelial cells in the liver and spleen. In the present study, the RES phagocytic index (RESPI) was measured in 48 subjects: 20 normal controls and 28 patients with DM. Measurements were based on the rate of disappearance (T1/2) of radioactivity in the heart (blood pooling) after rapid I.V. injection of 3mCi of Tc-99m phytate (2mg phytate). Using a gamma camera, sequential images of the liver and heart were obtained, and computer-generated time-activity curves were established. The T1/2 was then calculated. Our results revealed that the RESPI values in the DM group were significantly increased as compared to the normal control group (normal control = 4.60 +/- 1.01; DM = 6.78 +/- 3.83; P < 0.05). This prolonged half-disappearance time of blood radioactivity in diabetic patients is interpreted as a reflection of impaired phagocytotic efficiency in the reticuloendothelial system. The results of this preliminary study add support to the in vivo evidence explaining the increased susceptibility to infection in diabetic patients.