The ability to discriminate between stereoisomers of a-tocopherol was studied in five patients with abetalipoproteinemia (ABL) because an impairment in secretion of apolipoprotein B-containing lipoproteins might impede the normally enhanced plasma transport of RRR-a-tocopherol. An oral dose containing 3.7 g of each 2R,4′R,8′R-α-[5-C2H3]tocopheryl acetate (d3RRR-α-tocopheryl acetate) and 2RS,4′RS,8′RS-α-[5,7-(C2H3),]tocopheryl acetate (d6all rac-α-tocopheryl acetate) was administered, then the labeled and unlabeled a-tocopherol contents of plasma and red blood cells from multiple blood samples obtained at selected times up to 72 h following the dose were quantitated. ABL plasma contained about 1%-10% of the d3-RRR-a-tocopherol concentrations of normal subjects given only 150 mg of each isotope. Three of the patients discriminated between forms of a-tocopherol with ratios of RRR-/allrac-α-tocopherol ≥ 1.8, similar to normals. These data suggest that the hepatic tocopherol binding protein is present and functional in ABL patients. Although two of the patients did not discriminate between stereoisomers of a-tocopherol, it is likely that this resulted from nearly a complete block in very low density lipoprotein (VLDL) secretion. Thus, the ability of ABL patients to absorb and transport orally administered vitamin E is markedly impaired and variable among patients.