There are many investigations on the disturbances caused by x-rays but these works are consisted mainly of histo-morphological researches and rerely from the viewpoint of the acting mechanism of cells. More recently Lea, Barron et al, however, have studied the histochemical changes in the animals irradiated with x-rays and they have clarified the direct action of x-ray irradiation to a certain extent. In our laboratory Dr. Yamamoto has succeeded in isolating a certain cytotoxin from unsaturated fatty acid fraction, an extract of the liver of the rabbits exposed to x-rays. In the subsequent studies with this cytotoxin it has been found that this substance has an inhibitory action on the mitosis of tumor cells. Therfore, we have carried out a series of experiments to see the effect of this substance, in the presence of succinic dehydrogenase, on the respiration of Ehrlich-ascites-tumor cells. As the result it has been confirmed that this cytotoxin possesses an action as to depress both the respiration of the cancer cell and that of normal liver cells.