We studied the effects of recombinant human interleukin-1 beta (rhIL-1 beta) pretreatment on bleomycin (BLM)-induced pneumonitis in mice. Lung injury was dose-dependently induced by BLM (50 mg/kg, 100 mg/kg, 150 mg/kg i.v.). The mice were pretreated with IL-1 (1 microgram/mouse i.p.) at 0.5, 6, 12 or 24 hours before the administration of BLM. Wet lung weight, lung weight-to-body weight ratio and bronchoalveolar lavage cell findings were analyzed with respect to time, and lung specimens on day 28 after administration of BLM were histopathologically examined. When mice were pretreated with IL-1 at 0.5 hr or 6 hr before the administration of BLM, changes in all the parameters were significantly suppressed. The results indicate that IL-1 pretreatment protects mice from BLM-induced pneumonitis and its effects are time-dependent.