Myeloperoxidase (MPO), H2O2 and a halide form a powerful antimicrobial system effective against bacteria, fungi, viruses and mammalian cells. After phagocytosis, MPO is released into the phagosome from adjacent granules where it interacts with H2O2 generated either by leukocytic or microbial metabolism and a halide such as chloride or iodide to form agents toxic to the ingested organisms. Evidence for H2O2 and MPO participation in the microbicidal activity of polymorphonuclear leukocytes (PMNs) has been obtained from patients with neutrophil dysfunction. In chronic granulomatous disease, PMNs have a microbicidal defect associated with the absence of the respiratory burst. The importance of H2O2 deficiency in the PMN dysfunction is emphasized by its reversal by H2O2. PMNs which lack MPO also have a major fungicidal and bactericidal defect. Bactericidal activity is particularly low during the early postphagocytic period, after which the organisms are killed. Although emphasizing the importance of MPO-mediated antimicrobial systems particularly during the early postphagocytic period, these findings also indicate the presence of MPO-independent systems which develop slowly but are ultimately effective. The MPO-independent antimicrobial systems may be oxygen-dependent or oxygen-independent. The acetaldehyde-xanthine oxidase system has been used as a model of the MPO-independent, oxygen-dependent antimicrobial systems of the PMN. A microbicidal effect by this system was observed which was inhibited by superoxide dismutase, catalase and scavengers of hydroxyl radicals (OH') and singlet oxygen (1O2). The microbicidal activity of acetaldehyde and xanthine oxidase is increased considerably by MPO and chloride. The formation of ethylene from methional or 2-oxo-4-methylthiobutyric acid by PMNs has been regarded as evidence for OH' formation. We have found ethylene formation to be largely dependent on MPO and evidence for the initiation of ethylene formation by 1O2 has been obtained. Both the xanthine oxidase system and the MPO-H2O2-halide system convert diphenylfuran into cis-dibenzoylethylene, an effect which is compatible with, although not proof of, the formation of 1O2 by these systems.