Fetal wounds heal without scar formation, fibrosis, or contracture. Compared with adult wounds, they are characterized by major differences in the extracellular matrix and the absence of myofibroblastic cells. The reasons for these differences are not well known and determination of factors affecting the absence of scarring in the fetus may lead to strategies for controlling adult pathological scarring. In the present study, we have assessed the effects of serum on the behavior of normal human dermal fibroblasts. Using an in vitro approach, we investigated the effects of fetal and adult serum on cell properties such as growth rate, collagen synthesis, gelatinase activities, and differentiation to myofibroblasts using biochemical, morphological, and ultrastructural parameters. We studied the induction of alpha-smooth muscle (alpha-SM) actin in fibroblasts, and its correlation with increased collagen gel contraction by the cells. Our results showed that, compared with FBS (fetal bovine serum), postnatal calf serum (PCS) decreased mitogenic activity and collagenase synthesis but not collagen synthesis. Furthermore, cells cultured with PCS differentiated to myofibroblasts with an increase in cell diameter, number of stress fibers, alpha-SM actin expression, and collagen gel contraction. To characterize the molecules involved in this differentiation process, the amount of transforming growth factor beta (TGFbeta) in FBS and PCS was determined and the effect of neutralizing anti-TGFbeta antibody was evaluated. It was determined that FBS contained more TGFbeta than PCS, but that essentially all the TGFbeta was latent in both sera. However, results obtained with anti-TGFbeta antibody show that active TGFbeta is present when human dermal fibroblasts are cultured with medium containing PCS. These results suggest that, in the presence of PCS but not FBS, the cells either produce active TGFbeta or an enzyme that is able to activate latent serum TGFbeta. Alternatively, sera may contain two different forms of latent TGFbeta, the PCS form being activated by the dermal fibroblast cells. A similar mechanism may be involved, at least in part, in skin wound healing and may underlie the appearance of myofibroblasts in postnatal wounds.