Phagocytic activity of leukocytes in blood was examined in 70 patients with diabetes mellitus. 40 of them had insulin-dependent diabetes--(IDDM) or type I, while there were 30 patients with noninsulin-dependent diabetes (NIDDM) or type II. Phagocytic activity of leukocytes was determined by quantitative method of ingestion, on the principle of quantifying phagocytized fungi (Saccharomyces cerevisiae) and free phagocytes (nonphagocytizing leukocytes) by means of a phase-contrast microscope. The data have been statistically processed by the Student t-test and variance analysis test (Duncan test). The index of phagocytosis amounted to 3.2 +/- 0.77 in diabetic patients, while it was 3.47 +/- 0.29 in healthy examinees from the control group, thus yielding a statistically significant difference, p < 0.05. Out of all examined parameters (blood glucose, glycosylized hemoglobin, mean glucose value) phagocytic activity of leukocytes showed a statistically significant correlation with mean value of glucose in blood. Patients with mean glucose value higher than 12 mmol/l showed a significantly lower index of phagocytosis (2.9 +/- 0.84) than patients whose mean glucose value was lower than 12 mmol/l (3.5 +/- 0.59), p < 0.05. Neither age and sex of the patients nor chronic complications caused by diabetes affected the phagocytic activity of leukocytes in diabetic patients. There was no significant difference in the phagocytic activity of leukocytes between patients with IDDM and those with NIDDM.