The Carriers of Native Vitamin B₁₂ in Normal Human Serum

1. After fractionation of the vitamin B₁₂-binding proteins of ten normal sera the components containing transcobalamin II and R-type binders of vitamin B₁₂ respectively were studied for endogenous vitamin B₁₂ content by two distinct systems of vitamin B₁₂ assay.

2. The measurements of total serum vitamin B₁₂ by either bioassay with Euglena gracilis or a radioisotope dilution assay agreed closely.

3. The native vitamin B₁₂ carried by transcobalamin II was higher as measured by bioassay than by isotope dilution assay.

4. The presence of the transcobalamin II fraction of human serum altered the key reaction between the binding reagent of the isotope dilution assay, so that this assay failed to measure vitamin B₁₂ quantitatively.

5. Probably, the mean fraction of plasma vitamin B₁₂ carried by transcobalamin II is in the range 20–30%.