Serum vitamin B12and unsaturated B12binding capacity were measured in patients with a variety of hematological and nonhematological diseases. Both determinations were markedly increased in 25 patients with chronic myelogenous leukemia in relapse but decreased toward normal when clinical and hematological remission was achieved. However, values remained higher than normal in this disease even in its blastic phase and when treatment had rendered the marrow aplastic. Increases and decreases in the leukocyte count usually coincided with increases and decreases in the serum vitamin B12and binding protein values. The finding of an increased vitamin B12binding protein in patients with iron-deficiency anemia is unexplained.