Endotoxin causes pulmonary vascular neutrophil sequestration and injures the lung. Whether this is primarily due to a direct effect of endotoxin on the endothelium or is mediated by an action on the neutrophil is unclear. Canine neutrophils, isolated on plasma-Percoll gradients in vitro, were incubated with Salmonella enteriditis endotoxin, washed, and injected via wedged pulmonary arterial catheters into discrete lung zones of anesthetized dogs, whereas untreated neutrophils were administered into contralateral control lung zones. 113mIn-transferrin was administered intravenously 2 h before the animals were killed. Morphometry and extravascular protein accumulation were assessed at 4 h. Endotoxin treatment of neutrophils ex vivo induced a two- to three-fold increase in neutrophils in these lung zones (0.096 +/- 0.012 vs. 0.05 +/- 0.002 neutrophils/alveolar septal intercept, P less than 0.05). Extravascular-to-intravascular protein ratios in zones receiving endotoxin-treated neutrophils were significantly increased compared with control zones (0.146 +/- 0.02 vs. 0.079 +/- 0.02, P less than 0.05). Because complement fragments increase injury to endothelium in vitro, exogenous C5 fragments were administered to other dogs before administration of neutrophils but failed to significantly increase the extravascular protein signal (0.154 +/- 0.03 vs. 0.124 +/- 0.04). In summary, endotoxin treatment of neutrophils leads to neutrophil sequestration and increased pulmonary extravascular protein accumulation. C5 fragments failed to further enhance the protein accumulation. These data are consistent with an effect of endotoxin on the neutrophil to initiate neutrophil-endothelial interaction and subsequent lung injury.