A non-covalent complex of a 17 and a 7 kDa peptide was isolated from a Lys-C digest of the C-terminal 32 kDa half of the estradiol receptor by immunoadsorption. The 17 kDa part extends from K303 to K467, the 7 kDa part from S468 to K529 (or K531). The components are held together by hydrophobic interactions and can be separated by SDS/PAGE. They react on Western blots with MAB 13H2 (17 kDa) and MAB H222 (7 kDa), respectively. The native complex binds estradiol with high affinity and is recognized both by MAB 13H2 and H222, indicating that both peptides contribute to the ligand-binding niche.