A set of acute-phase proteins was found to have been induced in rat after triiodothyronine (T₃) treatment. To study the mechanism of gene activation by T₃, the rat gene of α1-acid glycoprotein (AGP), one of the major acute-phase proteins, was cloned and analyzed. CAT assay identified a thyroid hormone responsive element (TRE) in the first intron of the AGP gene and showed that mutations introduced into the TRE region destroyed the TRE activity. Gel shift assay and methylation interference assay showed that the thyroid hormone receptor bound to the TRE. The TRE identified here had a palindromic structure and was highly homologous to other known TREs.