Primary biliary cirrhosis is characterized by the presence of autoantibodies to mitochondria with specific reactivity to proteins of 74 and 52 kilodaltons (kd). The 74-kd mitochondrial protein is the E2 component — dihydrolipoamide acetyltransferase — of the pyruvate dehydrogenase complex, and the 52-kd protein is the equivalent E2 component — dihydrolipoamide acyltransferase — of the branched-chain α-keto acid dehydrogenase complex. Current methods for the detection of antibodies to these proteins lack specificity or sensitivity, or they are time-consuming and not readily available. We therefore developed an enzyme-linked immunoassay to quantify specific antimitochondrial antibodies in patients with primary biliary cirrhosis. Recombinant polypeptides coding for both the 74-kd and the 52-kd mitochondrial autoantigens were used to analyze 217 coded serum samples, including samples from 93 patients with primary biliary cirrhosis and 124 controls, for reactivity by our immunoassay, immunoblotting, and immunofluorescence testing. Serum samples from 89 of the 93 patients with primary biliary cirrhosis reacted with either the pyruvate dehydrogenase-E2 or the branched-chain α-keto acid dehydrogenase protein. None of the 124 control samples from healthy volunteers (n = 86) or patients with primary sclerosing cholangitis (n = 38) had significant reactivity. Our results indicate that the use of recombinant, cloned autoantigens provides a simple, accurate, and rapid method of quantifying and monitoring the levels of specific mitochondrial autoantibodies in the serum of patients with primary biliary cirrhosis. (N Engl J Med 1989; 320: 1377–80.), PRIMARY biliary cirrhosis is a well-characterized autoimmune disease of unknown cause that destroys the intrahepatic bile ducts of the liver. A characteristic serologic finding in patients with this disease is the presence of antimitochondrial antibodies that, at least by immunoblotting, are both highly directed and specific. Antimitochondrial antibodies from the serum of patients have been shown to react with at least three mitochondrial proteins with molecular weights of 74, 52, and 39. The serum samples of the majority of patients (80 to 95 percent) react with the 74-kd protein, and 30 to 50 percent react with the 52-kd protein. Less….