Although the field of study in immune enhancing compounds is relatively new, natural productsfrom plants represent a rich and promising source of novel molecules with immunomodulatingproperties. Microglial cells, the main immune effector cells of the brain, usually display a ramifiedmorphology and low expression levels of immunologically relevant antigens such as MHC class I andclass II. Since any compound which participates in activation of phagocytic cells contributes to theproduction of potentially toxic factors, the search for convenient in vitro test-systems and study ofmechanisms of action of these agents are of great interest. Human blood polymorphonuclear (PMN)cells and primary microglial cells isolated from Sprague-Dawley rats were used as cellular screeningtests for study of phagocytosis-stimulating action of immunomodulating agents. Numbers ofphagocytic activity were evaluated by the phagocyte ingestion of yeast cells and NO-synthaseactivity, nitrite production, and nitroblue tetrazolium test were determined after phagocytestimulation. It was possible to demonstrate that indexes of phagocytic activity can be used asquantitative indicators for measurement immunomodulating activity. As a positive control, ZymosanA-induced phagocytosis in both PMN cells and primary microglial cells was used. IFN-γ (0.1 - 1 U/ml)stimulated phagocytosis in PMN cells 1.2 times after 2 - 3 h incubation, although at higherconcentrations (10 - 100 U/ml) it strongly inhibited phagocytosis. In a similar way, at higherconcentrations, IFN-γ (100 - 500 U/ml) suppressed phagocytosis in zymosan-A stimulated microglialcells. When Polypodium leucotomus, cambricum and vulgare extracts were tested alone, increased levelsof phagocytosis were observed in PMN. In addition, microglial cells showed both increasedphagocytosis and MHC class-II antigen expressions. Surprisingly, when PMN and microglia weretreated with a combination of Polypodium and IFN-γ, phagocytosis was not inhibited. We did not findchanges in NO-synthase activity and nitrite production in both microglia and PMN cells activated bydifferent immunomodulating agents. These results indicate that primary microglial cell cultures aswell as human PMN cells can provide reproducible quantitative results in screening phagocyticactivity of different immunoactive compounds. Furthermore, both inhibitory or activationmechanisms might be studied using these in vitro experimental approaches.