Different doses of the isobutyl ester of retinoic acid were administered intraperitoneally into rats for two weeks. The dose of 1 mg/kg caused a significant increase in the mitotic activity of ceils of the macrophagal-s-eries in alveoli; an anti-inflammatory effect was observed at a dose of 10 mg/kg. The isobutyl ester of retinoic acid did not affect the cell ratio (alveolar macrophages, lymphocytes, neutrophils, eosinophils) in the internal medium of the lungs.