Followings are the results derived from the treatment of malignant tumors of animals with the fibroblast-inhibiting agent such as phytonadione based on the unique idea of ours. 1) No effect was observed on the catalase activity of liver tussue homogenate of normal healthy mice by the direct addition of excessive doses of phytonabione. 2) No change was observed in the liver catalase activity of normal mice abministrated 50mg/kg of phytonadione. 3) Inhibition of decreasing liver catalase activity was observed at two weeks after abministration of phytonadione in mice bearing sarcoma-180 ascites and subcutaneous tumors. 4) No inhibition of decreasing liver catalase activity was observed at one week after administration of 50mg/kg of phytonadione in mice bearing Ehrlich subcutaneous cancer. 5) With the combination use of phytonadione and one of the fibroblastinhibiting agents, chloroquine, a more remarkable inhibiting effect of decreasing liver catalase activity was observed than with a single abministration of chloroquine in mice bearing asrcoma-180 ascites tumor. 6) Remarkable carcinostatic effect was observed with the combination use of mitomycin-C and phytonadione than with the single administration of mitomycin-C.