Nucleic acid metabolism of tumor itself and liver in cancer bearing mice, DNA matabolism of intraperitoneal free cancer cells were measured by microspectrophotometer (MSP) and their correlation with cancer cell proliferation were analyzed under administrations of anticancer agents and their supporting substances. The results obtained are as follows: 1) The ratio of RNA to DNA is approximately 2.6 in liver of normal mice. DNA was considerably increased in cancer bearing group, and was about 2.5 times greater than that of normal group. 2) Mitomycin inhibited DNA metabolism of tumor, also inhibited DNA assimilation in liver of tumor bearing mice. Anticancer supporting substances, esfecially orotic acid, in contrast, could hardly cause any change in DNA metabolism of tumor, but slightly increased RNA metabolism. 3) With regard to nucleic acid metabolism in cancer cells, DNA inhibited cell counts were increased and octoploid cell counts were decreased in case of mitomycin administration. On the other hand, three peak type of cellular arrangement was obtained in case of the administration of supporting substances, showing disturbance of DNA assimilation process. 4) Mitomycin caused disturbance of cell proliferation in early stage of administration course, and also abnormal proliferation and cell destruction. The supporting substances, on the other hand, did not cause any disturbance in process of normal proliferation but accelerated the original degeneration process of tumor eclls as if to drive tumor cells to the final stage of necrosis.