The processes by which Ehrlich ascites tumor cells are killed were examined, and the mode of action of the cytotoxic factor was discussed. As a result, it was inferred that the cytotoxic factor was adsorbed on the surface of the target cells within 15min. and then transferred into the intracellular space through endocytosis mediated by a colchicine-sensitive structure. or microtubules. The cytotoxic factor which entered into the cytoplasm is expected to inhibit a metabolism in the target cells.