Antimitochondrial autoantibodies recognizing 68 to 74 and 50 to 52 kD inner membrane mitochondrial antigens are characteristically present in sera of patients with primary biliary cirrhosis. The biochemical identification of the antigens, however, has remained elusive. We report herein that the 52 kD antigen is the dihydrolipoamide acyltransferase of the branched‐chain α‐keto acid dehydrogenase complex. This was demonstrated by three experiments through the use of recombinant fusion protein expressed in Escherichia coli from a cDNA insert encoding the human autoantigen. First, 36 of 37 primary biliary cirrhosis patients exhibiting reactivity toward the 50 to 52 kD mitochondrial antigen by immunoblotting also showed reactivity toward the recombinant fusion protein. Second, absorption of primary biliary cirrhosis sera with recombinant fusion protein, but not with an irrelevant recombinant clone, the F‐specific rat liver antigen, was effective in absorbing out reactivity against the 50 to 52 kD mitochondrial antigen but not the 68 to 74 kD antigen. Third, complete removal of reactivity toward all four different isoelectric point polypeptides at 50 to 52 kD was observed in two‐dimensional gel analysis. Furthermore, primary biliary cirrhosis sera were analyzed with mitochondria from three sources, rat liver, human placenta and bovine heart, in order to compare reactivity patterns and to determine precisely the comparative molecular weights of the autoantigens in the three species. The availability of recombinant autoantigens will provide improved diagnostic tests and, more importantly, will allow definite issues in primary biliary cirrhosis to be studied, including identification of immunodominant epitopes, the significance of autoantigen recognition and the establishment of autoreactive T cell clones.