The effect of corticosteroid on an acute inflammation in the lungs was studied in guinea pigs exposed to an aerosol of bacterial endotoxin, the subsequent inflammatory response was evaluated counting the number of cells obtained from airway lavage and in the lung interstitium as well as the chemotactic effect of alveolar macrophages. Pretreatment with corticosteroid decreased the number of neutrophils in the airways at 24 hours after exposure but did not influence neutrophils in airways or lung interstitium at 4 hours after exposure, not the secretion of chemotactic factor(s) from alveolar macrophages at that time. It is suggested that the antiinflammatory effect of corticosteroids is a protective effect on the epithelium, preventing the late influx of serum fluid and neutrophils into the airways.