Recombinant rat gamma-interferon was found to inhibit collagen accumulation induced by the intratracheal administration of bleomycin in rat lungs. gamma-Interferon also elicited a reduction in histamine content in the lungs of the bleomycin-treated rats; however, this agent did not cause any obvious changes in inflammatory cell infiltration into the lung. These results suggest that gamma-interferon inhibits the development of pulmonary fibrosis by suppressing collagen synthesis and could thus be used as a therapeutic agent for preventing this condition.