Myofibroblasts characterized by alpha smooth muscle actin(alpha-SMA) expression play a key role in pulmonary fibrosis. Transforming growth factor-beta1 (TGF-beta1) is likely to be involved in the emergence of myofibroblasts, but the intracellular signal pathways for this process have not been well determined. The aim of the present study was to investigate the role of mitogen-activated protein kinase (MAPK)/activator protein-1 (AP-1) signaling pathways in TGF-beta1-induced alpha-SMA expression in human fetal lung fibroblasts (HLF-02). We found that TGF-beta1 treatment activated p38 kinase and extracellular signal-regulated kinase (Erk) in HLF-02 cells. The induction of alpha-SMA by TGF-beta1 was suppressed by p38 kinase inhibitor (SB203580) and Erk inhibitor (PD98059). AP-1 inhibitor curcumin also inhibited TGF-beta1-induced alpha-SMA expression. In addition, dominant negative mutant c-Jun (TAM67) downregulated TGF-beta1-induced AP-1 transactivation and alpha-SMA expression. In additional, PD98059 but not SB203580 inhibited the AP-1 DNA binding activity induced by TGF-beta1. Based on these findings, we conclude that p38 kinase, Erk, and AP-1 are responsible for the alpha-SMA expression induced by TGF-beta1 in human fetal lung fibroblasts. Erk is involved in inducing alpha-SMA expression via AP-1 activation.