Abstract
It has been shown that the addition of hydrogen peroxide to the incubation medium greatly increases the in vitro binding of tritiated estradiol or its degradation products to uterine tissue. The binding probably is covalent. It involves a hemoprotein which can be demonstrated histochemically by a peroxidase reaction. In light and electron microscopic radioautographs, the sites of radioactivity in uterine tissues after incubation with tritiated estradiol are mainly the specific granules of eosinophils and the cytoplasm of epithelial cells. The possible significance of an interaction between estradiol and a peroxidative hemoprotein is discussed.