A polyphenol extract from the aerial roots of the medicinal plant Geranium sanguineum L. (PC) inhibited the reproduction of influenza viruses type A and B in vitro and in ovo and protected mice from mortality in the experimental influenza infection. The in vivo protective effect was connected with multiple biological activities of the preparation. The present paper focuses on the in vitro effects of the polyphenol extract on the functions of peritoneal and alveolar macrophages and blood polymorphonuclear leucocytes (PMNs), isolated from healthy ICR mice. It was found that PC in doses of 12.5 and 25 microg ml(-1) stimulated the phagocytic activity of peritoneal macrophages and blood PMNs. PC in the same doses did not significantly affect the phagocytic activity of alveolar macrophages, the migration of alveolar and peritoneal macrophages or the adherent activity of PMNs. Used in concentrations of 3.1-25.0 microg ml(-1), PC suppressed spontaneous NO production from peritoneal macrophages, while inducible NO production, provoked by LPS-, Ifn-gamma and LPS + Ifn-gamma inductions was not affected. The cell-toxic concentration of 100 microg ml(-1) increased spontaneous and LPS-inducible NO production. The experimental results demonstrated a stimulating effect of PC on the phagocytic activity of murine PMNs and peritoneal macrophages as well as a beneficial effect of the preparation on spontaneous NO production.