ANTAGONISTIC GROWTH-ACTIVATING AND GROWTH-INHIBITING PRINCIPLES IN SERUM.

The substances precipitated by CO(2) from the serum of chickens 1 or 2 years old slightly increased the activity of pure cultures of a 10 year old strain of fibroblasts, an indication that a growth-promoting substance had been obtained from the serum. The rate of growth of fibroblasts was slower in serum deprived of its globulins and the increase of the inhibiting effect of the treated serum is thus manifested. It would appear that the inhibiting action of serum is referable to one or several growth-activating substances which precipitate by CO(2), and to one or several growth-retarding substances which remain in the supernatant serum after the precipitate has been thrown down by centrifugation. It may be concluded that, under the conditions of the experiments: 1. A substance enhancing the proliferative activity of homologous fibroblasts is precipitated from serum by CO(2). 2. The serum from which the precipitate has been removed inhibits the proliferation of homologous fibroblasts more markedly than does the original serum. The solution of the precipitate in the treated serum has about the same effect on fibroblasts as the original serum. 3. The restraining effect of serum on the activity of homologous fibroblasts is due in part to the antagonistic action of growth-activating and inhibiting substances.