Experiments were performed to clarify the circumstances under which a rat fibroblast-like cell line proliferated in culture or when injected into syngeneic hosts. Rat FR3T3 and SV40FR3T3 cells in culture multiply exponentially at comparable rates regardless of the horse serum concentration in the culture medium. The concentration of horse serum in the medium determines the length of the exponential phase of cell multiplication. Heat inactivation of the serum for periods as long as 24 hr does not impair the ability of the serum to sustain maximal cell multiplication rates. At low serum concentrations the exponential phase may be prolonged by refeeding the cultures daily with fresh medium. In medium supplemented with 50% fresh rat serum, FRT3T3 and SV403T3 cells are prevented from multiplying by yet undefined component(s) in this serum. This correlates well with the lack of tumor formation by these cells when inoculated into syngeneic hosts. The evidence obtained in culture strongly suggests that the multiplication ability of established cells in culture is a dominant constitutive character of these cells that is expressed whenever they are provided with optimal supply of nutrients and when no specific inhibitory conditions are present. Based on the data presented, we postulate that cell multiplication is a repressible function. These data also are compatible with a negative control mechanism for cell multiplication.