Although mitomycin C is known to have a remarkable antitumor effect, its practical application is often limited, because of the side toxic effects, particularly of those on the hematopoietic organ of the host^1~8). The purpose of the following series of experiments is to find out which schedule of administration would give a maximum antitumor effect with a minimum side effect. Mitomycin was given in various doses, either in consecutive or in intermittent injections, and either alone or in combination with carzinophilin, and then the antitumor effects and the side effects on the host animals were compared.