Vitamin E was administered orally (400 IU twice a day) to adult male humans for 28 days as either dl-alpha-tocopheryl acetate (all-rac-alpha-tocopheryl acetate) or d-alpha-tocopheryl acetate (RRR-alpha-tocopheryl acetate). Plasma alpha-tocopherol rose rapidly and fell at the same rate following cessation of supplementation with both forms of vitamin E. No significant differences in plasma alpha- or gamma-tocopherol levels were found between the two forms of vitamin E following their administration. The results confirm the currently accepted biopotencies of 1.0 IU/mg and 1.36 IU/mg, respectively for the two forms of vitamin E. Supplementation with either form of alpha-tocopheryl acetate resulted in depressing plasma gamma-tocopherol to less than 1/3 of initial levels; also the gamma/alpha ratio was depressed to less than 1/7 of the initial value. The study suggests that the gamma/alpha vitamin E ratio might also serve as a sensitive index of alpha-tocopherol ingestion.