In order to study the effect of X-irradiation on the composition of fatty acids in the body of cancer bearing animals, at first solid Ehrlich carcinoma was transplanted into the right leg of mice and then 500 R/day of X-rays were irradiated on the right leg of the animals consecutively for 10 days (total dosage 5,000R). The animals were divided into three groups of the irradiated cancer bearing mice, non-irradiated cancer bearing mice and normal mouse control group. The fatty acid extraction was carried out by the method of Folch et al with (1) tumor and right leg muscle, (2) serum, and (3) liver, of each group, and the determination of the fatty acid composition was made by the gas-liquid chromatography. 1. As the result it was found that the tumor tissue of the irradiated group revealed a greater quantity of linoleic acid than that of the non-irradiated cancer bearing group. 2. The solid Ehrlich carcinoma of the right leg contained a larger amount of stearic acid than that in the right leg muscle of normal mice. 3. As for the composition of total fatty acids in the liver and the serum, the percentage of linoleic acid was found greatest in (1) the control group of normal mice, followed by (2) irradiated tumor bearing mice, and (3) non-irradiated tumor bearing mice in the order mentioned. 4. There seems to be some correlation between the cessation of tumor growth by irradiation and the distribution of linoleic acid.