1. Both normal and sarcomatous fibroblasts of the rat utilize many different fragments of the protein molecule for their growth in vitro. Alpha and beta proteoses have approximately equal growth-promoting power.

2. A mixture of peptones, peptides, and amino acids, containing a negligible quantity of proteose, produces a temporary proliferation of normal fibroblasts, and an unlimited multiplication of sarcomatous fibroblasts, provided these substances are derived from liver which contains products of unknown nature that complete the nutritive effect of the protein degradation products.

3. Amino acids contribute to the nutrition of the cells, but are unable without the addition of peptides or polypeptides to support their life.

4. The proteolytic products are more toxic to normal than to sarcomatous fibroblasts. The hypothesis is suggested that the greater acidity produced by the large glycolysis of the sarcomatous cells may account for this difference through altering the speed of action of protein synthetizing enzymes.

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