This study was designed to quantify the role of reactive oxygen metabolites (ROMs) in two distinct components of murine peritoneal macrophage activity, phagocytosis and killing, and to discriminate quantitatively the degree to which each component is dependent on NADPH oxidase and/or xanthine oxidase. A fluorochromatic vital staining technique was modified to simultaneously quantify phagocytosis and microbicidal activity of macrophages incubated with Candida parapsilosis targets. To determine the role of ROMs, macrophages were preincubated with free radical scavengers [superoxide dismutase (SOD) and/or catalase] or with selective inhibitors of xanthine oxidase (XO, e.g., allopurinol) or NADPH oxidase [diphenyleneiodonium (DPI)]. Phagocytosis was not affected by treatment of macrophages with SOD, catalase, allopurinol, or DPI. Candidacidal activity, however, was inhibited by SOD, allopurinol, or DPI. The inhibitory effects of DPI and allopurinol were additive. Histochemical and biochemical assays demonstrated substantial quantities of XO in murine peritoneal macrophages. The findings suggest that the generation of ROMs by XO- and NADPH oxidase-dependent pathways are each important for phagocytic killing by murine peritoneal macrophages.