We have isolated an endogenous ligand which acts as a competitive inhibitor of the binding of Htamoxifen to triphenylethylene antiestrogen-binding sites (TABS) prepared from liver and from serum low density lipoproteins (LDL). This ligand is present in boiled ethanol extracts of rat liver and may represent an “endogenous antiestrogen”. “Endogenous antiestrogen” is used here as an operational term, since it has not been shown that TABS are involved in the mechanism of action of the triphenylethylene antiestrogens.