1. The pepsin hydrolytic products of the pure proteins, crystalline egg albumin, crystalline edestin, and purified fibrin, are utilized by fibroblasts for their proliferation. It appears, therefore, that the growth of fibroblasts in fibrin digests and in the proteose of Witte's peptone is due to the split products of the protein itself and not to accompanying cellular constituents or other impurities. Preliminary denaturation of the protein, as carried out in these experiments, does not alter the nutritive properties of the digest.

2. The digests of the pure proteins employed are deficient in certain substances and do not meet the entire nutritive requirement of the cells for an unlimited period of time. Some supplementary nutritive substances are present in fresh embryonic heart tissue. This circumstance explains the growth of fibroblasts from fresh embryonic heart in digests which do not promote the growth of a pure strain of fibroblasts.

3. Glycocoll and nucleic acid have been found to supplement the nutritive action of pure protein digests for sarcomatous fibroblasts, and to increase greatly the length of life of the tissues.

4. Vegetable proteins, as well as animal proteins, yield proteolytic products which promote the multiplication of fibroblasts.

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