The variation of the free fraction of serum vitamin B12 level was studied in certain hematologic disorders. In normals, the mean ratio of free to total vitamin B12 was 10.2 per cent. In leukemias—chronic myeloid, acute myeloid and acute lymphatic—the mean value of free vitamin B12 was higher than normal. In chronic myeloid and acute lymphatic leukemias, the free vitamin B12 level did not on an average exceed 20 per cent of the total. In acute myeloid leukemia, the free fraction was relatively more increased, constituting 31.1 per cent of the total on an average. In aplastic anemia, the mean values of free vitamin B12 and the ratio (31.2 per cent) of free to total vitamin B12 were higher than normal. The patterns presented by acute myeloid leukemia and aplastic anemia appeared to be more or less similar.