Pulmonary fibrosis, defined as the accumulation of connective tissue in the lungs, is a severe and often fatal form of interstitial lung disease. Transforming growth factor-beta (TGF-beta) is a powerful activator of connective tissue synthesis and fibroblast proliferation in the lung, and a critical paracrine signal for the development of pulmonary fibrosis. To investigate signaling pathways downstream of TGF- beta that contribute to lung fibrosis, TGF- beta stimulation of fibroblasts was replicated by treating NIH3T3 fibroblasts with conditioned medium (CM) from TGF- beta -treated type II alveolar epithelial cells (ATII cells). The data showed that fibroblast growth factor 2 (FGF-2) signaling is responsible for TGF-beta 1 CM-induced fibroblast proliferation, while it does not affect TGF-beta 1 CM-induced fibrotic differentiation. Moreover, fibroblast proliferation and differentiation induced by TGF- beta CM was totally abrogated by pretreatment of NIH3T3 cells with the specific ERK1/2 inhibitor, PD98059. These findings indicate that FGF-2 secreted by alveolar epithelial cells in response to TGF- beta 1 induces fibroblast proliferation and fibrotic activation through the ERK kinase pathway.