The effects of phagocytosis on the metabolic responses of rabbit alveolar macrophages have been studied in vitro. These cells exhibit peroxidative metabolism which is stimulated phagocytosis. One H2O2 generating oxidase, D-amino acid oxidase, has been demonstrated. Two H2O2 utilizing pathways exist, namely a nicotinamide adenine dinucleotide phospate (NADP) linked cytoplasmic glutathione shuttle and catalase-dependent reactions.