The concentration of lipid-soluble, chain-breaking antioxidants in human plasma and in erythrocyte ghosts have been determined for the first time by an inhibited-autoxidation method. The results are very similar to the concentrations of vitamin E measured for the same blood components by the HPLC method. It is concluded that vitamin E, which is largely present as alpha-tocopherol, is the only significant lipid-soluble, chain-breaking type of antioxidant present in human blood. The concentration of vitamin E in the plasma lipids divided by the concentration of vitamin E in the ghost membrane lipids is approximately a constant despite the large differences in vitamin E-intake and in plasma lipid concentrations in different individuals. Vitamin E/lipid ratios for plasma and ghosts were larger for subjects taking a supplement of alpha-tocopherol acetate of 100 IU per week, compared to nonsupplemented subjects (based on data from a limited number of subjects). A larger supplement of 2800 IU per week did not significantly increase the vitamin E/lipid ratios.