The growth of muscle fibers during late development as well as in regeneration following muscle injury is the result of the proliferation and differentiation of satellite cells. However, all human cells, including satellite cells, show a limit in their proliferation. In order to define a cellular system with enhanced proliferative capacity, human satellite cells were transfected with a construct containing large T antigen from SV40 under the control of the human vimentin promoter. Vimentin is normally expressed during proliferation, and its expression is down-regulated as differentiation proceeds. In transfected cells, the construct is regulated like the endogenous vimentin gene. The effect of exogenous T antigen expression on both the proliferation and differentiation of human satellite cells was investigated. T antigen expression reduced the doubling time of human satellite cells from 36 to 20 h and increased the final proliferative capacity from 46 to 69 mean population doublings. When differentiation was triggered, although T antigen did not prevent the formation of myotubes, fusion was delayed. A similar delay was observed in the appearance of myogenin protein, one of the HLH regulatory factors, but not in the corresponding mRNA. Finally, T antigen has an effect on adult myosin isoform expression, since both adult slow and fast isoforms were only detected in myotubes negative for T antigen. These results led us to propose a model of the possible interactions between T antigen and muscle-specific factors.