Most of the sera from patients with primary biliary cirrhosis contains antimitochondrial antibodies, which react with four proteins of the mitochondrial inner membrane. We reported in a previous paper that when beef heart mitochondrial inner membrane proteins were digested by trypsin, a new reactive 36 kDa fragment with antimitochondrial antibody was obtained. This 36 kDa fragment derives from original 70 kDa protein because the monoclonal antibody specific to 70 kDa protein reacts with the 36 kDa band equivalent to 70 kDa band. The 36 kDa fragment was purified using an affinity column conjugated with an immunoglobulinrich fraction of primary biliary cirrhosis serum containing antimitochondrial antibody, preparative electrophoresis and high-performance liquid chromatography using a reverse phase column. The final preparation showed a single band in sodium dodecyl sulfate polyacrylamide gel electrophoresis. Its amino acid composition is in good agreement with that of the subunit binding domain of the pyruvate dehydrogenase complex E2 from bovine heart.