The effect of sphingosine 1-phosphate (S1P) on the proliferative response to low serum was examined in two closely related cell populations, such as C2C12 reserve cells and myoblasts. S1P reduced DNA synthesis promoted by serum in myoblasts, whereas it enhanced the mitogenic response to serum in reserve cells. By employing selective S1P receptor agonist and antagonists, the co-mitogenic action of S1P in reserve cells was shown to depend mainly on S1P(1). Real time PCR analysis revealed distinct S1P receptor pattern expression, which however could not account for the differential action of S1P in C2C12 reserve cells and myoblasts, thereby suggesting that the cell-specific responsiveness to S1P may depend on a different functional coupling of S1P(1). This study discloses a unique pleiotropic effect of S1P which stimulates proliferation of muscle resident stem cells, such as reserve cells, and favours the growth arrest of committed progenitors cells, such as myoblasts, required for their subsequent myogenic differentiation.