Microglia are the most important immune cells within the highly specialized environment of the central nervous system. Upon activation, they transform from a resting 'ramified' into a fully functional 'amoeboid' phenotype with the ability to perform phagocytosis and generate free radicals. A combined flow cytometric assay for the simultaneous measurement of these two functions in porcine microglia in vitro is presented: reactive oxygen species are detected using hydroethidine; phagocytosis is assessed using fluorescein isothiocyanate-labeled latex beads. The combination of these two probes allowed us to distinguish four subpopulations within cultured porcine microglia on the basis of their functional activity. The effect of several exogenous stimuli [phorbol myristate acetate, conditioned medium, interferon γ (IFN-γ)] on the in-vitro functional properties of porcine microglia is investigated using this test. In particular for IFN-γ, a significant modulatory effect on the intracellular reactive oxygen species production and phagocytic activity was observed. This result suggests an alternative role of IFN-γ acting on cultured porcine microglia.