Polyoxidonium (PO) is a high-molecular weight physiologically active compound with pronounced immunomodulating activity, an N-oxidized polyethylene-piperazine derivative. The aim of our work was to study cellular and molecular mechanisms of the action of PO on the human peripheral blood leukocytes. By means of flow cytometry it was established that the binding of fluorescein-isothiocyanate-labeled PO (FITC-labeled PO) occurs more rapidly with monocytes and neutrophils than with lymphocytes (7- to 8-fold weaker as compared with monocytes). Using colloidal gold-labeled PO and electron microscopy it was shown with that the preparation penetrates into leukocytes by endocytosis. PO is localized in endoplasmic vesicles of cellular cytosol. Analysis of one of the crucial signal transducer, the intracellular Ca 2+, performed with the Fluo-3 fluorescent dye, showed that PO does not induce Ca 2+ mobilization from the intracellular calcium stores and influx of extracellular Ca 2+. The study of the intracellular hydrogen peroxide (H 2O 2) production with the 2′,7′-dichlorfluorescein indicator demonstrated that PO significantly increases the level of intracellular H 2O 2 in monocytes and neutrophils, however, this increase is much less as compared with phorbol myristate acetate stimulation. The analysis of immunomodulating effect produced by PO proved its stimulating activity on some cytokines production in vitro, e.g. interleukin 1β (IL-1β), tumor necrosis factor (TNF)-α and IL-6. A dose-dependent increase in the intracellular killing by blood phagocytes was established under the action of PO. © 2004 Elsevier B.V. All rights reserved.