1. Serum from normal subjects has been chromatographed on Sephadex G-200 columns and the fractions containing transcobalamins 0, I and II have been identified.
2. The fractions corresponding to transcobalamin I contained, on average, 90% of the endogenous vitamin B12. Only 3% was attached to transcobalamin 0, and 7% was bound to transcobalamin II.