The increase in the lipid peroxide level in the serum and liver of female mice after bilateral ovariectomy evidenced antioxidant activities of female hormones. This increase was abolished upon administration of female hormones. Similar increase in the level of lipid peroxide was observed in the serum of women who had undergone bilateral ovariectomy. Injection of 2-hydroxyestradiol suppressed the increase in the lipid peroxide level in the liver of rats receiving whole-body gamma-ray irradiation. Considering that the mechanism of coronary atherosclerosis is ascribed at least in part to the increased level of lipid peroxides, estrogen therapy could be applied to women who had undergone bilateral ovariectomy prior to menopause or to normal women after menopause. 2-Hydroxyestradiol might be applied also to men.