Primary biliary cirrhosis (PBC) is characterised by the presence of antimitochondrial antibodies. The PBC-specific, immunoreactive, trypsin-sensitive antigens on the inner mitochondrial membrane (M2) have hitherto not been identified. A major 70 kD M2 autoantigen is the E2 component (lipoate acetyltransferase) of the pyruvate dehydrogenase enzyme complex located within mitochondria. This has been confirmed by immunoblotting of PBC patients' sera against purified E2 protein: sera from 38/40 (95%) patients with established clinical, biochemical, and histological features of PBC (18 stage II/III, 22 stage IV) reacted positively with E2; whilst no sera from 39 controls (27 non-PBC chronic liver disease, 12 healthy normal women) gave a positive response. Immunoblotting showed that a second subunit of the pyruvate dehydrogenase complex, a 50 kD polypeptide of unknown function (component X), is also an M2 autoantigen. Identification of these M2 mitochondrial antigens should facilitate the development of a specific serological test for PBC and the study of autoimmunising epitopes.