Abstract The effect of posthepatectomy and normal rat or human serum on cultured cells derived from rat hepatocytes was studied. Hepatectomies (70 percent), sham hepatectomies, and unilateral nephrectomies were performed in rats, and serum obtained 24 hours later was added to the culture medium. Addition of serum (5 and 10 percent) from hepatectomized rats caused considerable stimulation of cell growth; this effect was not altered when serum from rats which underwent hepatectomy and unilateral nephrectomy was added to the medium. Serum from normal, sham-hepatectomized or unilaterally nephrectomized rats produced no effect on cell growth. Similarly, serum from normal human beings or from a patient who had undergone a negative laporotomy following trauma produced no effect. In contrast, serum (5 and 10 percent) from a man 14 days following 75 percent hepatectomy done because of liver injury stimulated cell growth considerably. Initial attempts to isolate the stimulating factor revealed that the active compound(s) is entirely filtrable through cellophane membrane and has molecular weight (MW) of less than 12,000. We conclude: (1) normal serum does not contain an inhibitor of cell growth, (2) a factor stimulating cell growth in vitro is present in the serum of rats and man following partial hepatectomy, and (3) this stimulating factor has a MW of less than 12,000.