Elevation of transcobalamin I and serum vitamin B12 levels has usually been associated with increased granulocytic proliferation, such as occurs in chronic myelogenous leukemia. Two patients with metastatic cancer had extremely high serum vitamin B12 and transcobalamin I levels--greater than those seen in even the most intense granulocytic proliferation--that were not explainable by leukocytosis. The subjects' serum vitamin B12 levels were 18,750 and 21,221 pg per milliliter (normal, 471 plus or minus 174 pg per milliliter, mean plus or minus S.D.) and unsaturated vitamin B12 binding capacity 158,750 and 5,400 pg per milliliter (normal, 1153 plus or minus 313 pg per milliliter) respectively. The abnormally elevated serum binder was shown to be identical with transcobalamin balamin I in every respect. Levels of transcobalamin II and serum third binder were normal. The cause of the binder abnormality is unknown, but factors other than granulocyte proliferation may control or contribute to the production or accumulation of transcobalamin I.