The affinity for thyroid hormone receptor (TR) of polybromodiphenyl ethers (PBDEs) and hydroxylated PBDEs was examined. 4-Hydroxy-2,2',3,4',5-pentabromodiphenyl ether (4-OH-BDE-90) and 3-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (3-OH-BDE-47) markedly inhibited the binding of triiodothyronine (1×10^-10 M) to TR in the concentration range of 1×10^-6-1×10^-4 M. 2,3,4,5,6-Pentabromophenol (PBP) also showed an inhibitory effect at 1×10^-5-1×10^-4 M. However, 2,2',3,4,4',5'-hexabromodiphenyl ether (BDE-138), decabromodiphenyl ether (DBDE), 4-methoxy-2,2',3,4',5-pentabromodiphenyl ether (4-MeO-BDE-90), 4'-hydroxy-2,2',4,5'-tetrabromodiphenyl ether (4'-OHBDE-49), 4-hydroxy-2,2',3,4'-tetrabromodiphenyl ether (4-OH-BDE-42), 4'-hydroxy-2,2',4-tribromodiphenyl ether (4'-OH-BDE-17), 3'-hydroxy-2,4-dibromodiphenyl ether (3'-OH-BDE-7), 2,4,6-tribromophenol (TBP) and tetrabromohydroquinone (TBHQ) did not show affinity for TR. In contrast, 4'-OH-BDE-17 and 3'-OH-BDE-7 exhibited estrogenic activity in estrogen-responsive reporter assay using MCF-7 cells at the concentration of 1×10^-5 M. However, adjacent bromo substitution of 3- or 4-hydroxylated PBDEs markedly decreased the estrogenic activity. These results suggest that hydroxylated PBDEs act as thyroid hormone-like agents, as well as estrogens, that a 4- or 3-hydroxyl group in PBDEs is essential for thyroid hormonal and estrogenic activities, and that adjacent dibromo substitution favors thyroid hormonal activity, but not estrogenic activity.