Intravascular activation of the complement system with cobra venom factor results in acute lung injury, which has been quantitated by increases in lung vascular permeability. Cobra venom factor preparations devoid of phospholipase A2 activity retain full lung-damaging capacity. The lung injury is associated with the preceding appearance of chemotactic activity in the serum coincident with the development of a profound neutropenia. The chemotactic activity is immunochemically related to human C5a. Morphologic studies have revealed discontinuities in the endothelial cell lining of lung alveolar capillaries, damage and/or destruction of endothelial cells in these areas, plugging of pulmonary capillaries with neutrophils that are in direct contact with vascular basement membrane, the presence of fibrin in alveolar spaces and in areas adjacent to damaged endothelial cells, and intraalveolar hemorrhage. Lung injury is dramatically attenuated in animals that have been previously neutrophil depleted. Teh intravenous injection of superoxide dismutase or catalase also provides significant protection from the pulmonary damage. Very little protection from the pulmonary damage. Very little protection is afforded by pretreatment of rats with antihistamine. These studies suggest that intravascular activation of the complement system leads to neutrophil aggregation and activation, intrapulmonary capillary sequestration of neutrophils, and vascular injury, which may be related to production of toxic oxygen metabolites by complement-activated neutrophils.