Description: The mitochondrial antigen M2 is a cluster of four major mitochondrial innermembrane proteins of approximately 74, 52, 45, and 39 kD, which are found widely in nature. (2) These antigens have been identified as the E2 subunit of branched chain 2-oxo acid dehydrogenase, the E2 subunit of the 2-oxo dehydrogenase complex, protein X, and the E1a and E1b subunits of the pyruvate dehydrogenase complex (3). Autoantibodies to mitochondrial antigens were originally described in patients with Primary Biliary Cirrhosis. It was later realized that Primary Biliary Cirrhosis is an autoimmune liver disease characterized by autoantibodies to the mitochondrial antigen M2 in serum (1). The presence of autoantibodies against the mitochondrial antigens has been reported to have prognostic value in the diagnosis Primary Biliary Cirrhosis, as the disease rarely occurs without anti-M2. (2) Autoantibodies to M2 in Primary Biliary Cirrhosis usually occur in a very high titer and have an IgG3 subclass restriction (2).