['Human polymorphonuclear leukocytes (PMN) produce nitric oxide (NO), superoxide (O2.-) and peroxynitrite (ONOO-) upon stimulation. We investigated the role of ONOO- in PMN-induced injury to cultured bovine aortic endothelial cells (BAEC).', 'BAEC were cocultured with phorbol 12-myristate 13-acetate (PMA)-activated human PMN (effector-to-target ratio, 10:1) and injury to BAEC was evaluated at intervals by 51Cr release assay. The levels of NO, O2.-, ONOO- and nitrotyrosine, a reaction product of ONOO-, were also measured, and the influence of NO synthase inhibitors, O2.- and hydroxyl radical scavengers and other effectors was examined.', 'In BAEC cocultured with PMA-activated PMN, 51Cr release was significantly increased [14.6 +/- 2.2% at 2 h (p < 0.05) and 42.6 +/- 2.7% at 4 h (p < 0.01); control (nonactivated PMN), < 4%]. Superoxide dismutase (100 U/ml) reduced 51Cr release to 4.6 +/- 2.2% at 2 h (p < 0.05). N-Iminoethyl-L-ornithine (L-NIO, 0.1 mM) potentiated 51Cr release (30.6 +/- 3.8% at 2 h, p < 0.01), and the potentiation was eliminated by anti-CD18 monoclonal antibody. The 51Cr release was completely prevented by dimethyl sulfoxide or by deferoxamine. Treatment of PMN with L-NIO inhibited NO generation and increased O2.- production. The nitrotyrosine level did not increase in BAEC cocultured with PMA-activated PMN.'] Is peroxynitrite a major mediator of endothelial cell injury by activated neutrophils in vitro?