In view of the fact that there may be two possible actions in radiation distrurbaces in vivo, namely, one acting directly and the other acting indirectly, and on the assumption that these actions, persisting quite a long time after irradiation, induce specific substance in the irradiated nimals, Yamamoto of our department, has succeeded in isolating unsaturated fatty acid substance (OX) specific to the irradiated animals. With the purposa to find out what influences this substance (OX) will have on Ehrlich's ascites tumor cell, the author studied its effect on the number, mitotic phases and DNA ontents of the cancer cells. Test animals used are 80 mice each wighing about 15-20g. To these animals 0.2cc cell suspension of Ehrlich's ascites tumor cell is transplanted intraperitoneally and one week after the transplantation 0.2cc of 2%-, 1%-, 0.5%- OX substance is injected intrapeitoneally to these animals. By observing the changes in the cell counts, mitotic phase, and the DNA contents in each cell (with spectrophotometry) at the intervals of 1, 3, 6, 9, 12, and 24 hours after the injection of OX substance, and the following results were obtained: The number of cancer cells has been found markedly decreased; nomely, after one hour it is down to 78 per cent of the original count; down to 44per cent after 9 hours; and clown to 46.2 per cent after 24 hours. As for the mitotic phases, metaphase, anaphase telophase have been increased and the number of cells undergoing mitosis is diminished. Moreover, as for the DNA contents per cell, there can be observed an increase ni the number of cells with a markedly small amount of DNA, while the cells undergoing duplication due to mitosis have been found decreased in number. From these facts it is concl ded that OX substance acts on Ehrlich's ascites tumor cells as to induce the following disturbances: (1) cell destruction, (2) impairment of the activity in mitotic apparatus, (3) disturbance of the DNA synthesis, and (4) elicits atypical cell divistion.