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Hepatocyte growth factor (HGF) is a growth factor for alveolar epithelial cells. Activation of pro-HGF to HGF is regulated by the HGF activator (HGFA), a serine protease, and a specific inhibitor (HGFA inhibitor-1, HAI-1). An imbalance in the HGFA/HAI-1 system might contribute to lung fibrosis. Pro-HGF activation capacity from bronchoalveolar lavage (BAL) fluid was evaluated 3, 7, and 14 days after the intratracheal bleomycin injection (Bleo) in mice with or without thrombin. BAL fluid from naive mice was used as control. HGFA and HAI-1 mRNA were evaluated by QPCR in the whole lung or by Western blot in BAL fluid. BAL fluid from control mice and Bleo mice activated pro-HGF in vitro at a similar degree. Thrombin accelerated proHGF activation by Bleo BAL on Day 3 and Day 7, but not on Day 14, or in control BAL. Incubation of pro-HGF with BAL from Bleo Day 3 and Day 7 mice increased phosphorylation of HGFR on A549 cells. Thrombin-induced pro-HGF activation was inhibited by an anti-HGFA antibody and accelerated by an anti-HAI-1 antibody. Active HGFA was not detected in control BAL and was strongly induced in Bleo BAL. HGFA concentrations were higher on Day 3 and Day 7 than on Day 14. HAI-1 was detected at low levels in control BAL and increased strongly by Day 3 with stable concentrations until Day 14. By demonstrating an imbalance between HGFA and HAI-1 expression in BAL fluid, our results highlight a defective thrombin-dependent proHGF activation system at the fibrotic phase of bleomycin-induced pulmonary fibrosis.