3-Hydroxyacetaminophen has been isolated and identified as a microsomal metabolite of acetaminophen. Analysis of the metabolite by gas chromatography-mass spectrometry revealed that the metabolite had a molecular ion and fragmentation pattern identical to that of authentic 3-hydroxyacetaminophen. Glutathione and ascorbic acid blocked covalent binding of reactive metabolite(s) to protein but did not block the formation of 3-hydroxyacetaminophen. Moreover, epoxide hydrolase did not block covalent binding of the reactive metabolite(s) to protein. Thus, the reactive metabolite apparently is not an epoxide substrate of the hydrolase, nor are 3-hydroxyacetaminophen and the reactive metabolite(s) formed from a common intermediate.