Clinical and laboratory evidence suggests that the fulminant liver failure sometimes associated with the inhalation anesthetic halothane may be an immunemediated toxicity. Most importantly, the vast majority of patients with a clinical diagnosis of halothane hepatitis have serum antibodies, which react with one or more specific liver microsomal proteins that have been covalently altered by the trifluoroacetyl chloride metabolite of halothane. The serum antibodies are specific to halothane hepatitis patients and are not seen in sera of patients with other types of liver pathology. In this study, a 57-kD trifluoroacetylated liver microsomal neoantigen associated with halothane hepatitis and native 57-kD protein were purified from liver microsomes of halothane-treated and -untreated rats, respectively. When the purified trifluoroacetylated 57-kD and native 57-kD proteins were used as test antigens in an enzyme-linked immunosorbent assay, serum antibodies from halothane hepatitis patients (n = 40) reacted with both of these proteins to a significantly greater extent than did serum antibodies from control patients (n = 32). On the basis of its apparent monomeric molecular mass, isoelectric point and NH2-terminal amino acid and tryptic peptide sequences, the 57-kD protein has been identified as rat liver protein disulfide isomerase. Antibodies raised against rat liver protein disulfide isomerase also reacted with a protein of approximately 58-kD in human liver microsomes. The results of this investigation suggest that trifluoroacetylated protein disulfide isomerase is one of the immunogens associated with halothane hepatitis. In certain patients it might lead either to specific antibodies or, possibly, to specific T cells, which could be responsible for halothane hepatitis. (HEPATOLOGY 1993;18:858-863).