High levels of estradiol (E2) were detected by radioimmunoassay (RIA) in supernatants of rat thymus and spleen, kidney, lung and lung macrophages cultured in the presence of fetal bovine serum(FBS). A competitive inhibitor of E2 synthesis and no effect on the production of E2 by thymic cultures. Acid hydrolysis of estrogen conjugates in FBS released enough free E2 to account for the high level found in culture supernatants. Removal of free estrogens and estrogen conjugates by charcoal stripping the serum before addition to media of rat tissues by hydrolysis of estrogen conjugates in FBS.
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