A comparative study was made on the uptake of intra-peritoneally administered radioiron by catalase of blood, liver, and, bone marrow in guinea pigs and rabbits. Radioactivity of (59)Fe per catalase activity (c. p. m./K cat/mg protein), viz., specific activity, was greatest in marrow, followed by liver and blood. Maximum incorporation of (59)Fe in liver catalase was attained on the 6th day, and in marrow catalase on the 9th day. In blood catalase, however, specific activity continued to increase gradually through the entire experimental period. These data showed that blood catalase is different from liver catalase in its origin and originates, probably, in bone marrow, And it was demonstrated by these rapid turnover of (59)Fe that in liver and bone marrow catalase was, labile. Catalase solutions, then, were evaluated by means of starch block electrophoresis. In each catalase, which is recollected from starch block after electrophoresis, (59)Fe incorporation curve was similar to the results above described, so it was made clear that origin of blood catalase is different from that of liver catalase. Last, some studies were made on the inhibition of catalase activity by X-radiation on mice to cofirm, mainly, that blood catalase originates in bone marrow. On the rate of decrease of catalase activity after X-radiation, there were obvious difference between blood and liver. Activity of liver catalase decreased about 20% at the beginning and about 40% at the end of this experiment. On the contrary, the rate of decrease of catalase activity of blood was slight only about 5%. On the last day of the experiment (the 12th), not only catalase activity of blood but hemoglobin cancentration showed sudden depression which indicates the destruction of bone marrow by X-radiation. From these facts, it proved that the origine of blood catalase and liver one are different, and that blood catalase originates in bone marrow.