Two polymorphic variants in the gene coding for transcobalamin II (TCN2), TCN2 776C- > G and TCN2 67A- > G, may alter serum holotranscobalamin (holoTC), which in turn may affect cellular uptake of cobalamin (Cbl) and thereby Cbl status indicators. We studied the effects of TCN2 776C- > G and TCN2 67A- > G on blood concentrations of holoTC, Cbl, methylmalonic acid (MMA), and total homocysteine (tHcy) in 2411 individuals (50-64 y) that had been selected on the basis of these TCN2 genotypes from 10601 Norwegian inhabitants. The serum holoTC concentration was lower in TCN2 67AG (55 ± 0.75 pmol/L) and 67GG (48 ± 2.14 pmol/L) than in 67AA (62 ± 0.67 pmol/L) (P < 0.001) but did not differ among TCN2 776C- > G genotypes. The polymorphisms interacted as serum holoTC determinants (P = 0.001) and the presence of TCN2 67AG and GG in strata of 776CC and CG, but not 776GG, increased the risk of having serum holoTC < 45.6 pmol/L [tertile 1 vs. tertiles 2 and 3: OR = 2.5 (95% CI 1.8-3.5) for 67AG; OR = 5.7 (95% CI 3.5-9.1) for 67GG in 776CC; OR = 2.1 (95% CI 1.6-2.9) for 67AG; and OR = 4.5 (95% CI 2.4-8.2) for 67GG in 776CG; all P < 0.001]. Plasma MMA, tHcy, and Cbl were not affected by either polymorphism. In summary, serum holoTC, but not plasma Cbl, MMA, or tHcy, varied according to TCN2 67A- > G genotypes. It remains to be determined whether this polymorphic effect on serum holoTC alters its diagnostic utility as Cbl status indicator.