1. Twelve patients, 6 with pernicious anemia and 6 without pernicious anemia as controls, were studied as to urinary excretion of vitamin B12.*
2. Microbiologic tube assay using Lactobacillus leichmanii A.T.C.C. # 4797 was the method of B12 assay.
3. Dosages used were 42.2, 63.3, 84.4, 211.0 µg given both in ascending and descending order at daily intervals.
4. Amounts of 84.4 to 211.0 µg of vitamin B12 parenterally resulted invariably in the excretion of 53 to 68 per cent of the injected vitamin B12 within 18 hours. The percentage of excretion rose remarkably as the dosage increased.
5. An amount of 42.2 µg of vitamin B12 parenterally resulted in relatively little excretion when this was the initial dose, but the excretion was appreciable when these doses followed the injection of the larger doses. "Saturation" of the body apparently occurs. In general more vitamimi B12 is excreted in the smaller doses if these follow the larger doses.
6. No significant differences could be detected between the pernicious anemia and nonpernicious anemia patients.
7. A possible explanation of these results is proposed.