1. The inhibiting action on homologous fibroblasts of the heated serum of a young animal increases relatively more than that of an old animal. After it has been heated, the inhibiting action of the serum of the old animal is still greater than that of the young animal.

2. The CO₂ precipitate obtained from the serum of a young animal definitely increases the proliferative activity of homologous fibroblasts, while the CO₂ precipitate of the serum of an old animal has practically no activating power. After the removal of the CO₂ precipitate, the inhibiting action of the serum of young animals is increased, while that of old animals is not modified.

3. The increased inhibiting action of serum on homologous fibroblasts in old age is partly due to the disappearance of the growth-activating substances and to the enhanced activity of the growth-inhibiting principle.