A patient with recurrent pulmonary abscess, weight loss, and alcoholism was found to have extremely high serum vitamin B12 and unsaturated vitamin B12-binding capacity (UBBC) levels. While transcobalamin (TC) II was also increased, most of his UBBC was due to an abnormal binding protein which carried greater than 80% of the endogenous vitamin B12 and was not found in his saliva, granulocytes, or urine. This protein was shown to be a complex of TC II and a circulating immunoglobulin (IgGkappa and IgGlambda). Each IgG molecule appeared to bind two TC II molecules. The reacting site did not interfere with the ability of TC II to bind vitamin B12, but did interfere with its ability to transfer the vitamin to cells in vitro. The site was not identical to that reacting with anti-human TC II antibody produced in rabbits. Because of this abnormal complex, 57Co-vitamin B12 injected intravenously was cleared slowly by the patient. However, no metabolic evidence for vitamin B12 deficiency was demonstrable, although the patient initially had megaloblastic anemia apparently due to folate deficiency. The course of the vitamin B12-binding abnormalities was followed over 4 yr and appeared to fluctuate with the status of the patient's illness. The IgG-TC II complex resembled one induced in some patients with pernicious anemia by intensive treatment with long-acting vitamin B12 preparations. The mechanism of induction of the antibody formation in our patient is unknown.