Fetal calf serum (SVF), insolubilized on a sepharose matrix, retains mitogenic factors (FM) from horse serum (SC) which can be eluted by molar acetic acid. The FM can: 1 degree initiate the synthesis of DNA in serum starved, contact inhibited 3T3 BALB/c fibroblasts and 2 degrees allow at least three generations to take place. The FM have no antitryptic activity and do not allow cell attachment. Weight to weight, this fraction is 10 to 50 times more active than horse serum. Polyacrylamide gel electrophoresis shows that various proteins are present, particularly immunoglobulins. The intervention of epidermal growth factor (EGF), fibroblast growth factor (FGF) or the platelet-derived growth factor (PDGF) have been excluded.