A guinea pig model was developed to document the association of polymorphonuclear neutrophils (PMN) decrease with pulmonary leakage after a sublethal infusion of endotoxin (LPS Escherichia coli 0111:B4, 5 µg/min/kg). A control group was prepared in the same manner and given an infusion of vehicle alone. Severe leukopenia developed, mostly expressed in the neutrophil population. There was a decrease in RBC catalase and fibronectin. Plasma leakage in the lung followed the endotoxin infusion, which could indicate endothelial cell damage. The results from the present study suggest an activation of PMN in endotoxin-infused animals, degranulation and release of toxic oxygen species, resulting in pulmonary plasma extravasation