Activated granulocytes have been implicated in mediating pulmonary endothelial damage in the Adult Respiratory Distress Syndrome. In another lung disease, emphysema, pulmonary granulocytes (PMNs) are thought to be doubly responsible for lung dissolution: they release potent proteolytic enzymes including elastase, and they generate reactive oxygen species that oxidize a reactive site methionine group in alpha-1-protease inhibitor (α-1-PI) rendering it, in turn, impotent as an anti-elastase. This suggested an analogous scenario for pulmonary vascular damage: namely, undefended PMN elastase might also mediate endothelial injury. Our strategy to prove this notion used 51chromium-labeled human endothelial cells exposed to intact PMN or to enucleate 'neutroplasts'. The latter are elastase-free cytoplasmic blebs derived from PMN. When activated, both PMN and neutroplasts generate similar amounts of toxic oxygen species; yet neutroplasts caused insignificant endothelial damage, measured as 51Cr 'lift-off' from anchoring matrix (PMN = 24.3% ± 1.8% vs neutroplast = 1.2% ± 0.4%; p < 0.001). Adding pure elastase back to neutroplasts increased endothelial cell lift-off (7% ± 0.2%). Although the prototypic serine protease inhibitor phenyl methylsulfonylfluoride (PMSF) protected endothelium from PMNs, pure α-1-PI (also a potent anti-elastase) when added in physiologic amounts did not protect endothelial cells from PMN assault, suggesting that PMN oxidants might inactivate it. By adding exogenous myeloperoxidase (MPO) to MPO-deficient neutroplasts, we demonstrated that MPO-dependent oxidants, probably N-chloramines, are critical inactivators of α-1-PI. This was further confirmed since added free methionine, a scavenger of chloramine, protected α-1-PI from inactivation by reagent chloramine or that produced by rearmed neutroplasts or PMN. Concomitantly, free methionine inhibited PMN-provoked endothelial cell lift-off in a dose-dependent fashion while conjointly defending anti-elastase activity of α-1-PI. We conclude that stimulated neutrophils can injure vascular tissue by combining elastase secretion with oxidative inactivation of the anti-elastase, α-1-PI.