1. A pure strain of fibroblasts has been isolated from the Jensen rat sarcoma. The cells give rise to tumors on transplantation into animals and during several months of life in vitro have maintained their malignancy unchanged.

2. The malignant cells are generally coarser and more refringent than normal cells. They possess the cytological characteristics of fibroblasts without showing any morphological abnormality. They can be considered as healthy cells. The texture of their colonies is looser than that of normal fibroblasts and Sarcoma No. 10 fibroblasts. Their residual activity does not differ markedly from the normal. They proliferate unlimitedly in a nutrient medium.

3. They liquefy the fibrin of rat plasma and turn phenol red golden yellow. They do not liquefy the fibrin of chicken plasma.

4. They multiply in chick embryo juice, calf liver digest, and also in rat serum. Their growth activity is increased by the presence of bone marrow.

5. The unlimited growth of Jensen sarcoma within the body may possibly be attributed to the ability of the fibroblasts to maintain themselves upon the substances present in rat serum. This property itself probably depends upon the increased enzyme activity of the cells.

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