Bleomycin-induced lung disease is characterized by fibroblast proliferation and pulmonary fibrosis. In these studies, we demonstrate that fibroblasts are relatively resistant to clinically relevant amounts (below 10(-4) U/ml) of bleomycin and that these levels of bleomycin augment fibroblast proliferation in response to various fibroblast growth factors. These observations suggest that one mechanism by which bleomycin causes pulmonary fibrosis is augmentation of fibroblast proliferation.