Transfection of C3H/10T1/2 cells with either a c-myc or an activated c-Ha-ras gene decreased the cellular dependence for serum-derived factors to proliferate in monolayer. The c-myc-transfected cells did, however, require a high plasma concentration for significant growth, while the ras transfectants grew extremely well in either low or high concentrations of either plasma or serum. Stimulation of quiescent cultures with purified growth factors demonstrated that c-myc transfection did not alter qualitatively or quantitatively the requirement for both epidermal growth factor (EGF) and insulin to progress to DNA synthesis. Cells transfected with either a ras gene alone or a combination of ras plus c-myc lost their dependence on EGF for DNA synthesis; cultures became committed to S phase in serum-free medium supplemented with insulin alone. The ras transfectants arrested in mid-G1, 6 h prior to S phase. The EGF independence of the ras transfectants is consistent with the mid-G1 arrest of these cells at a point(s) distal to the primary action of EGF in early G0-G1.