We report a new method for measurement of holo-transcobalamin (holoTC), in which magnetic beads coated with vitamin B12 (cobalamins) precipitate apo-transcobal-amin (apoTC) and the holoTC present in the supernatant is measured by ELISA. Serum holoTC denotes the part of vitamin B12 accessible for the cells of the body and is considered to be a sensitive marker of vitamin B12 deficiency (1–3). Serum holoTC is not easily measured because it accounts for only approx-imately one-third of the circulating vitamin B12 and be-cause the major part of TC circulates unsaturated with vitamin B12 (apoTC) (1–3). In the few published attempts to measure holoTC, TC is separated from the other vitamin B12-binding protein, haptocorrin, before quantifi-cation of vitamin B12. This allows a direct measurement o
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