In normal mouse fibroblasts exponentially growing at suboptimal serum concentration the population of cells having a presynthetic (G1) DNA content is heterogeneous, since there are present both competent and non-competent cells (Sturani et al, 1984). We show here that SV40 transformed 3T3 fibroblasts, which have lost growth control and serum requirement, lack the G1 non-competent phase. These findings are confirmed by the different kinetics of exit from the G1 compartment of transformed and normal cells. In addition a positive correlation is reported between the fraction of non-competent cells and the average abundance of the phosphorylable PDGF receptor on the cell surface of normal cells.