1) An attempt was made to study on an anti-tumor agent, nitromin, based on its medication period against subcutaneously transplanted ascites hepatomas MH 134 to C(3)H/HeN (♂) X dd (♀): F(1) mice. 2) The earlier administration of nitromin after the tumor transplantation would inhibit the tumor growth more effectively. When the administration was continued from the 3rd day after the transplantation, some cases could be found to survive more than a year. 3) The earlier nitromin administration would extend the survival period in 50 % the cases. The administration of nitromin that was continued from the 10th day after the transplantation can hardly inhibit the tumor growth; in contrast, the administration which was started within the 7th post-transplantation day appears to be effective to inhibit the tumor growth. 4) The superficial lymphnode metastases were observed in cases of the long survived mice, that is, those which received the anti-tumor agent in early stage. It does not mean the ineffectiveness of the agent, but the result of the long survival with the agent. 5) The lung metastasis tends to occur in cases of the late administration of the agent.