We attempted to reserch that a poison are produced in blood when the blood are irradiated, and whether such poison causes tissue injuries secondarily or not. Experimental Method: After the rabbits blood of removing serum in test tube are irradiated with great doses of X-rays or Co(60) at a time, such blood injected to other healthy rabbits, and then we research to what change are brought in such case, and we find out following matter. Experimental Results: The peripheral blood cell count reduced after transfusions, the function of the reticuloendotherial system falled and the apperance of Heinz's body erythrocytes were remarkable. In view of these facts, it seems to be produced a toxic substance in the peripheral blood of the rabbit irradiated with X-ray or Co(60), and the the toxic substance causes radiation injuries secondarily.