The effect of polyoxidonium, a synthetic immunomodulator, which is a water soluble high-molecular compound, an N-oxidized polyethylene-piperazine derivative, on the functional activity of human peripheral blood phagocytic cells was studied in vitro. It was established that a one-hour incubation of leukocytes with polyoxidonium increased the ability of leukocytes to kill the ingested Staphylococcus aureus in a dose-dependent manner. This increase is observed with leukocytes obtained both from healthy persons and from patients with chronic granulomatous disease. The study of spontaneous and stimulated chemiluminescence of phagocytes showed a significant decrease in the quantity of chemiluminescent impulses in extracellular space in the presence of polyoxidonium both in luminol- and lucigenin-dependent chemiluminescence. Polyoxidonium proved to have antioxidant activity at all doses tested (100, 250, and 500 μg/ml). The analysis of intracellular hydrogen peroxide (H 2O2) level by means of dichlorofluorescein, a fluorescent indicator, showed that incubation with polyoxidonium resulted in a higher luminescence intensity of dichlorofluorescein, thus indicating an increase in the intracellular H2O2 level. This increase is not so considerable as it is in case with phorbol myristate acetate stimulation. However with polyoxidonium used at a dose of 500 μg/ml, the difference with the control is significant both for neutrophils and monocytes. Polyoxidonium can be used as adjuvant in combined treatment of acute and chronic infections of any etiology, in the treatment of CGD and secondary immunodeficiencies simultaneously with basic drugs.