1. A media containing all the essential constituents for the cultivation in vitro of sarcomatous fibroblasts of the rat has been prepared by digesting calf liver and also the anterior lobe of the pituitary body with pepsin.
2. The nutritive action of the pituitary digests is not altered by thorough extraction with ether.
3. After a pure strain of sarcomatous fibroblasts had been cultivated for 3 months in a liver digest, its proliferative activity was as great as at the beginning of the experiment. The same was true of the colonies cultivated for 1 month in a digest of pituitary gland. The increase in the volume of the colonies which takes place in the digests is about as great as that produced by chick embryo juice.
4. Normal chicken fibroblasts also proliferate in both digests, but they undergo fatty degeneration after a more or less prolonged period of cultivation.