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1. Studies of the fate of intravenously injected radioactive vitamin B12 have been performed in patients with normal, low and high serum concentrations of vitamin B12. 2. Abnormal plasma disappearance curves were noted in chronic myelocytic leukemia, pernicious anemia in relapse and in remission, total gastrectomy and malabsorption syndrome. 3. In chronic myelocytic leukemia, the slow clearance of plasma radioactivity may be explained by the increased binding capacity of the plasma proteins for vitamin B12. 4. Plasma clearance of radioactivity is slower than normal in pernicious anemia, even in remission. The failure of the disappearance curve to return to normal in pernicious anemia in complete remission suggests the existence of a plasma "B12-transferase," whose function is to transfer circulating B12 to the tissues. The disappearance curves suggest that the amount of such "B12-transferase" is diminished in pernicious anemia, total gastrectomy and certain Cases of malabsorption syndrome. 5. A relationship between a hypothetical "B12-transferase" and intrinsic factor is discussed.

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