['The mechanism by which stimulated neutrophils (polymorphonuclear leukocytes [PMNs]) damage pulmonary vascular endothelium was investigated.', 'The ability of unstimulated and mechanically stimulated PMNs to adhere to pulmonary endothelial cells and, thereby, alter pulmonary vascular permeability was tested. Each series was conducted on 6 rats. To stimulate PMNs, they were agitated gently in a glass vial for 10 seconds.', 'Perfusing lungs with the stimulated PMNs elicited a fivefold increase in permeability compared with lungs perfused with the unstimulated cells. This increase in permeability was blocked completely by preincubation of stimulated PMNs with CD18 monoclonal antibody. This increase in permeability was also blocked completely by superoxide dismutase (SOD) or the xanthine oxidase (XO) inhibitor allopurinol. Pulmonary vascular hemodynamics were unaffected by any treatment protocol. The accumulation of stimulated PMNs within the lungs was not inhibited by SOD but was partially blocked by allopurinol.'] Does superoxide possibly produced in endothelial cells mediate the neutrophil-induced lung injury?