Reactive oxygen metabolic products derived from an activated NADPH oxidase present in the cell membrane of PMNs and mononuclear phagocytic cells play a critical role in the host's defense against bacterial infection. Recent studies have also demonstrated the ability of these toxic products to initiate eukaryotic cell injury and promote the development of the acute inflammatory responses. Experimental studies suggest that neutrophil-derived oxygen metabolites contribute to the development of the tissue injury associated with a variety of disease states, including emphysema, myocardial infarction, adult respiratory distress syndrome, immune complex-mediated vasculitis, and rheumatoid arthritis. Future studies to define further the mechanisms by which reactive oxygen-derived metabolic products mediate tissue injury will provide insight into the development of new therapeutic strategies for the modulation of disease states that are mediated by the recruitment and activation of PMNs.