The vitamin B12 binding capacities of transcobalamin I and II were evaluated in plasma from patients with diseases known to affect the plasma binding. In CML in relapse, the capacity of TC II was very much diminished or absent. The capacity of TC I was very much increased. There was a lesser degree of both abnormalities during remission and in AML. There was either diminished or absent binding to TC II in PA but the capacity of TC I was not altered.