Asthma is a disease of airway inflammation and bronchoconstriction that results in airway smooth-muscle cell hypertrophy and hyperplasia. The underlying mechanisms that induce myocyte proliferation remain unknown. Evidence suggests that cytokines such as transforming growth factor (TGF)-β1 may play a role in modulating this process. In this study, we examined the effects of TGF-β1 on human airway smooth-muscle (HASM) cell proliferation. We found that treatment of HASM cells with TGF-β1 inhibited epidermal growth factor (EGF)- and thrombin-induced DNA synthesis. This inhibition was both dose and time dependent. We then investigated whether these effects are mediated through activation of mitogen-activated protein kinase (MAP kinase), an enzyme that is thought to play a central role in the regulation of cell proliferation. We found that MAP kinase activation induced by EGF was not modulated by TGF-β1, despite TGF-β1 inhibiting EGF-induced HASM cell growth. These data suggest that TGF-β1 inhibits mitogen-induced HASM cell proliferation, but does so downstream from MAP kinase activation, or via a parallel pathway that is independent of MAP kinase activation.