1. The plasma disappearance of a small intravenous dose of radioactive vitamin B12 was determined in control subjects and in patients with various blood disorders.

2. A delayed, sometimes irregular, disappearance was observed in the majority of patients with acute and chronic myelogenous leukemia, myeloid metaplasia, and polycythemia vera.

3. Disappearance was normal in the lymphogenous leukemias, secondary polycythemia and relative polycythemia.

4. The abnormalities observed are believed to indicate an abnormality of vitamin B12 metabolism common to the diseases of the myeloproliferative group and are further evidence of the close relationship between these diseases.

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