Keratinocyte growth factor (KGF) and hepatocyte growth factor (HGF. are known mitogens for normal alveolar Type 2 cells in vitro and in vivo. We wished to determine whether these two growth factors are involved in lung repair after epithelial cell necrosis by determining the levels of each factor in lung lavage fluid collected serially after bleomycin-induced injury, and how these values relate specifically to proliferation of bronchiolar and alveolar epithelial cells. Rats received an intratracheal injection of 1 unit bleomycin in 0.5 ml water and were killed at intervals up to 4 weeks with 1 μCi/g tritiated thymidine injected 1 hour before death. Early necrosis of bronchiolar epithelial (BR) cells and Type 1 alveolar epithelium was followed by an increase in inflammatory cell numbers and high protein levels in bronchoalveolar lavage (BAL) fluids. In addition, the levels of KGF and HGF, measured by enzyme-linked immunosorbent assay in BAL, increased as early as 3 days and peaked at 7–14 days, when KGF was measured at 160 pg/ml (n = 50) and HGF reached 460 pg/ml (n = 40). Both values dropped sharply after 2 weeks. Epithelial cell proliferation was quantitated as percentage of labeled cells in autoradiographs of methacrylate sections. Labeling of BR cells predominated in the first week and peaked at 7% at 3 days. Type 2 cell proliferation was delayed somewhat but occurred in 3 to 10 days with a peak of 7% labeled cells at 1 week. The results demonstrate that both HGF and KGF are present in the lung in greatly increased amounts soon after bleomycin-induced epithelial cell necrosis. These high levels are associated with both BR and alveolar epithelial cell proliferation.