The development of bleomycin-induced pulmonary fibrosis was studied in rats depleted of neutrophils for 1 week and in rats depleted of complement for 1 week. Lung injury was assessed by histologic evaluation and by the biochemical measurement of total lung collagen. Qualitatively, histologic evidence of damage induced by bleomycin showed that damage progressed in a similar manner in neutropenic and complement-depleted animals when compared with normal animals. However, analysis of total lung collagen at 1 week after bleomycin administration revealed that collagen deposition was increased in neutropenic animals and decreased in animals depleted of complement when compared with untreated animals that received bleomycin. There were no significant differences in lung collagen levels at 1 month after bleomycin treatment in neutrophil-depleted or complement-depleted animals when compared with animals that received bleomycin alone.