The emergence of the myofibroblast phenotype (characterized by alpha-smooth muscle actin expression) in wound healing and in tissues undergoing fibrosis is thought to be responsible for the increased contractility of the affected tissues. In bleomycin-induced pulmonary fibrosis, the myofibroblast is also responsible for the observed increase in collagen gene expression. To evaluate further these phenotypic changes in lung fibroblasts, contractile and other phenotypic properties of fibroblasts isolated from lungs of rats with bleomycin-induced fibrosis were compared with those of normal rats using in vitro models. Pulmonary fibrosis was induced in rats by endotracheal injection on day 0, and 7 and 14 days later the animals were sacrificed and lung fibroblasts isolated. Using immunofluorescence, < 10% of fibroblasts from control animals express alpha-smooth muscle actin when cultured as a monolayer. In contrast, 19% and 21% of cells from day 7 and day 14 bleomycin-treated animals, respectively, expressed this actin and with greater intensity than in control lung cells. This increase in actin expression was associated with enhanced contractility when evaluated using a three-dimensional cell culture model consisting of fibroblast-populated collagen gels. This enhanced contractility was abolished by treatment with antibody to transforming growth factor-beta (TGF-beta), whereas exogenous TGF-beta 1 and serum-stimulated contraction of control lung fibroblasts. TGF-beta 1 gene expression was greater in cells from bleomycin-treated animals than those from control lungs. These results show that cells with the myofibroblast phenotype are more abundant in fibrotic lung, and that these cells possess greater contractile capacity in vitro at least partly by virtue of their enhanced endogenous TGF-beta 1 gene expression.