Neutrophils are thought to play a key role in tissue injury. We investigated the role of human neutrophil-derived elastase in the induction of injury to human pulmonary artery endothelial cells. Incubation of endothelial cells with neutrophils increased the release of lactate dehydrogenase activity, thrombomodulin, and preloaded fura-2 from endothelial cells, indicating that neutrophils induce endothelial cell injury. Attachment alone of neutrophils to endothelial cells appeared to induce activation because elastase release and N-formyl-mentionyl-leucyl-phenylalanine (fMLP)-induced superoxide (O2) production from neutrophils incubated with endothelial cells were greater than from neutrophils only. When endothelial cell were incubated with neutrophils stimulated by fMLP or phorbol myristate acetate, the amount of elastase in the medium and endothelial cell damage was further enhanced. However, when neutrophils were blocked from direct attachment to endothelial cells using a membrane filter, endothelial cell damage was ameliorated, while exogenous neutrophil elastase and medium containing neutrophil-released elastase did not induce endothelial cell injury. An inhibitor of neutrophil elastase, ONO-5046 Na, as well as erythromycin, which reduces neutrophil-derived elastase, dramatically inhibited neutrophil-induced endothelial cell injury. Superoxide dismutase (SOD) partially inhibited injury. Injury was completely inhibited by treatment with a combination of ONO-5046 Na and SOD. These results suggest that attachment of neutrophils to endothelial cells is important for endothelial cell damage and that neutrophil-derived elastase plays an important role in endothelial cell injury in combination with O2. In addition, ONO-5046 Na and erythromycin may be useful in treating diseases worsened by excessive neutrophil activity.