Guinea pigs were sensitized with anti-egg-albumin rabbit serum with various intervals after the whole body x-irradiation. Twenty-four hours later 0.1cc of antigen of 1, 0.1, 0.01 and 0.001% were injected on the abdominal skin intracutaneously. Arthus' phenomenon was observed after twenty four hours. The results were as follows: 1) By 200r x-irradiation, Arthus' phenomenon was mostly restrained on the 7th day after the exposure and returned to normal later but showed a tendency to incraese on the 28th day. The area of skin-redness at Arthus' phenomenon became small on the 7th day after exposure, and returned to normal later but showed a tendency to enlarge on the 28th day after the exposure. 2) By 400r irradiation, Arthus' phenomenon was mostly restrained on the 7th day and the area of skin-redness was scarecely observed. 3) Arthus' phenomenon by the method with Evans-blue injected intraveously as an index showed the largest area of the skincolouring on the 7th day after 200r x-irradiation without showing any clear margin. It returned normal afterwards. On the 28th day, the area of the skin-colouring was small with a clear margin. 4) After the 200r exposure, while the epidermis group was atrophic, pathological changes of skin were not so marked in general.