Our purpose is to understand why mdx muscle does not show the progressive degeneration observed in human Duchenne muscular dystrophy (DMD) muscle. In the mouse, the regenerative process compensates for the necrosis of the muscle fibers, particularly during the acute phase of the disease (5-9 weeks). In DMD muscle, there is a gradual failure of the regenerative process and the muscle fibers are replaced by connective and fatty tissue. We propose that distinct properties of mdx and DMD muscle fibroblasts could be one of the reasons for the differences between the mdx and DMD phenotypes. We found that fibroblasts taken from human DMD and control muscle had similar in vitro proliferative capacities. The proliferation rate of mouse muscle fibroblasts decreased during the acute phase of the disease, and inhibition was complete in fibroblasts from 8-week-old mdx mice. Moreover, the medium conditioned by these cells inhibited fibroblast proliferation. The effect was specific for fibroblasts, since this conditioned medium stimulated myoblast proliferation, as did control fibroblast-conditioned medium. These results suggest that 8-week-old mdx mouse muscle fibroblasts produce an inhibitor of their own proliferation and a growth factor specific for myoblasts in vitro. If these factors are secreted in vivo, the growth inhibitory factory may stop fibroblast proliferation whereas the mitogenic activity could stimulate satellite cell proliferation, thus favouring muscle regeneration.