A dialyzable component from the aqueous extracts of mouse skeletal muscle and liver inhibited in vitro growth of a mouse Ehrlich ascites. A similar component was not detected in extracts of spleen, kidney, lung, skin, serum or small intestine. The muscle component appeared to be different from that of the liver in its resistance to heat and its stability in the culture medium. Both components however were stable on storage at 4, 23 and 37°C for 72 h. Intraperitoneal injection of the muscle and liver component into mice previously innoculated with Ehrlich ascites significantly decreased tumor incidence in these animals as compared with the control