Estrogens are potent antioxidants both in vitro and in vivo. In this study the antioxidant affect of estradiol-17beta (estradiol) was compared with those of fat-soluble antioxidants (alpha-tocopherol and beta-carotene) in terms of both fatty acid (thiobarbituric acid-reactive substances and diene conjugation) and cholesterol oxidation (oxysterols). The addition of alpha-tocopherol (54 micromol/L) inhibited low-density lipoprotein (LDL) oxidation by 92.6% and high-density lipoprotein (HDL) oxidation by 76.5%. In similar experiments, estradiol (54 micromol/L) inhibited LDL oxidation by 77.5% but inhibited HDL oxidation by only 55.4%. Beta-carotene had no antioxidant effect. Lag times (diene conjugation method) for alpha-tocopherol and beta-carotene increased by 175% and 125%, respectively. Estradiol markedly reduced the maximum formation of diene conjugates as compared with results with alpha-tocopherol and beta-carotene, and it exhibited a linear curve (no change in lag time). In terms of cholesterol oxidation, estradiol was far more effective than alpha-tocopherol or beta-carotene in inhibiting oxysterol formation (microg/ml plasma) (control = 24.56 +/- 2.31, beta-carotene = 20.59 +/- 3.32, alpha-tocopherol = 20.19 +/- 1.58, estradiol = 14.38 +/- 0.70). This study shows that estradiol is as effective an antioxidant as alpha-tocopherol in terms of fatty acid peroxidation but is far more effective than alpha-tocopherol in terms of cholesterol peroxidation.