We developed a rapid and sensitive two-color flow cytometric method which allows the simultaneous quantification of both the phagocytosis rate and the oxidative burst activation of polymorphonuclear leukocytes (PMNLs). The oxidation of hydroethidine (HE) to ethidium bromide (EB) was performed by the oxidative neutrophil products within the cells during the respiratory burst, which was stimulated by phagocytized fluorescein-labeled Staphylococcus aureus. By means of flow cytometry we measured red EB fluorescence emission together with green fluorescence, which was emitted by the ingested fluoresceinated bacteria. The fluorescence intensity was proportional to the number of bacteria ingested. Adherent bacteria were distinguished from the ingested ones. This two-color cellular staining permits measurement of two different functions of neutrophils in one step. This method could be of interest for the determination of the interactions between neutrophils and bacteria and for the investigations on infectious diseases in chronic granulomatous disease patients.