Abstract Acetaminophen is metabolized by cytochrome P450 to a reactive metabolite that covalently binds to proteins and this binding correlates with the hepatotoxicity. The major protein adduct was previously reported to be a 55 kDa protein that was detected on Western blots using antisera specific for 3-(cystein- S-yl)acetaminophen. In this study, the 55 kDa protein was isolated using a combination of ion exchange fast flow chromatography, hydroxyapatite HPLC and anion exchange HPLC. Amino acid sequences of 8 internal peptides from a trypsin digestion of the 55 kDa protein were found to have 97% homology with the deduced amino acid sequence from a cDNA that corresponds to a 56 kDa selenium binding protein. This is the first report of a specific protein to which a metabolite of acetaminophen covalently binds.