Preventive measure of the side-effects of an anticancer agent, Mitomycin-C (MMC), was investigated on Ehrlich tumor mice, by using several agents which had originally no tumoricidal effects except last one: such as orotic acid (OA), chloroquine orotate (QO), magnesium chloride (Mg), Sarvinal (SV), dihydrostreptomycin (SM), and so-called OX-substance (OX; unsaturated fatty acid fraction extracted from the liver of X-Ray irradiated rabbit). Results obtained were as follows. 1. Mg had a most preventive effect on the reduction of WBC due to MMC, following by SM, CO and SV. 2. On the fluctuation of RBC, SM, Mg, OA and CO showed better results. 3. No agents tested were effective on the prevention of reduction in platelets. 4. No unwilling decrease in tumor-effects were noted in these agents. 5. Survival time was markedly extended in case of using SM. Summarizing the results obtained above, streptomycin was a most useful agent for the prevention of side-effects of Mitomycin-C. Under the fundamental investigation, streptomycin was clinically used in case of giving Mitomycin-C, and the side-effects were remarkably reduced as seen in the experiment. Further studies would have to be continued on the other anticancer agents.