The rat-fetal-resorption test currently is used to assess the biological activity of vitamin E compounds. Previous studies in humans, however, suggest that rat assays underestimate the potency of free tocopherol relative to the acetate ester form and of RRR-alpha-tocopheryl acetate relative to all-rac-alpha-tocopheryl acetate. Therefore, we studied the serum concentrations of alpha-tocopherol after ingestion of RRR-alpha-tocopheryl acetate, all-rac-alpha-tocopheryl acetate, RRR-alpha-tocopherol, RRR-alpha-tocopheryl succinate, and RRR-alpha-tocopheryl acetate plus apple pectin by each of 20 adult human subjects. Measurements were made at 0, 8, 24, and 48 h after ingestion of 800 IU of the various preparations. The results at 24 h were representative of the differences observed. The mean increase in concentration of alpha-tocopherols (mg/g lipid) in 24 h was 71.2% after RRR-alpha-tocopherol, 63.3% after RRR-alpha-tocopherol acetate, and 41.2% after RRR-alpha-tocopherol succinate. Animal assay data do not correlate with data from studies of absorption and retention in serum of alpha-tocopherols ingested by humans.