The contribution of neutrophils to the action of endotoxin on plasma exudation in the airways of anaesthetized guinea-pigs was quantified by measuring the extravasation of Evans blue dye. Endotoxin (Salmonella enteritidis) caused a dose-dependent increase in microvascular leakage to Evans blue dye which was maximal after 25 min (p less than 0.05). The minimum dose tested that induced a significant rise in leakage was 1.5 mg.kg-1 for "central" intrapulmonary airways (ipa); 4.5 mg.kg-1 for trachea and main bronchi and 7.5 mg.kg-1 for nasal mucosa, larynx and "peripheral" ipa. Depletion of circulating neutrophil numbers by 97% using an antibody to guinea-pig neutrophils caused no significant diminution of the effects of endotoxin on leakage in any part of the airway. There was no significant influx of neutrophils into the airway interstitium at the time of maximum extravasation of Evans blue. We conclude that endotoxin-induced airway microvascular permeability is dependent upon mechanisms other than circulating neutrophils.