We used an in vitro model to investigate whether macrophages stimulate satellite cells proliferation. Satellite cells were obtained by tryptic digestion of adult muscle. Macrophages were obtained from peritoneal cavity by wash after injection of thioglycolate broth. Macrophages and satellite cells cocultures showed an increased number of differentiated myotubes as compared to control cultures. Moreover, in conditions of myoblast colony growth, the addition of macrophage-conditioned medium resulted in a greater number of muscle cell colonies, which are richer in large and differentiated myotubes. The experiments with macrophage-conditioned media suggest that the increased muscle cell proliferation and differentiation is mediated by soluble factor(s) released by macrophages. These results demonstrate that besides their scavenger role macrophages play a pivotal role in myoblast proliferation during muscle regeneration.