In 1925 Fisher found that the growth of Rous sarcoma was acceralated by adding fibrobalsts into the culture medium of the sarcoma. Takeuchi found that the rapidly growing tissue of Brown Pearce cancer contained much mucopolysaccaride and he suggested that mucopolysaccaride had acceralated the tumor growth. On the basis of these findings, the author has attempted a few experiments as follows: Experiment 1. Chondroitin sulfate was injected into Strong A mice either subcutaneously or intraperitoneally. After this pretreatment, Ehrlich ascites tumor cells were transplanted to the back of mice. The tumor growth was observed until they were killed on the 12 th day after the transplantation. No signigicant difference in the tumor weight was found between the control group and the group which had been pretreated by the intraperitoneal inlection. A significant difference in the tumor weight, however, was found between the control group and the group given the subcutaneous pretreatment (P<0.01). Experiment 2. By using the same method, some other substances (extracts of L-strain cells and of mouse fetus, and P. V. P.) were studied. No intraperitoneal injection was tried this time. A significant difference in the tumor growth was found between the control group and the group which had been pretreated with the extracts of L-strain cells and mouse fetus, but not between the controls and the group pretreated with P. V. P..