1. The growth of chicken heart fibroblasts in physiological saline media, e.g. Tyrode, is not materially increased by the addition of pure proteoses.
2. In the presence of plasma, the addition of proteoses causes increased growth of the tissues, probably owing to the slow digestion of the proteoses to utilisable amino acids by the enzymes of the plasma.
3. If small quantities of embryo extract are also present in the plasma, then the beneficial effect of the proteoses on the rate of growth is very greatly increased.
4. A thermostable substance has been separated from Witte's peptone, andfrom fibrin digests, which is capable of stimulating cells to increased activity and division. The activity of this substance is not dependent on the presence of plasma.The effective agent is associated with, or is actually, a heteroproteose, and its action is probably not to supply nitrogen, but rather to stimulate the cells in some other way. It is present in certain preparations of proteoses and partly accounts for their activity.
5. Suggestions are put forward to account for the manner in which the various media described affect the rate of growth of the cells.