1. Sera from 76 patients with primary biliary cirrhosis (PBC) and 66 control subjects (53 with chronic liver disease and 13 healthy normal women) were immuno-blotted against purified E1 component of bovine pyruvate dehydrogenase complex (PDC) and bacterial PDC. 2. Thirty-one out of seventy-six (41%) sera from PBC patients showed a positive response to bovine E1 alpha, and five of these 31 (7% of total) reacted with bovine E1 beta. None of the control sera reacted with bovine E1 alpha or beta. 3. None of the PBC sera that recognized bovine E1 subunits reacted with bacterial PDC E1. 4. In the PBC patients there was no correlation between presence of antibodies to E1 alpha and beta subunits and histological stage of the disease. 5. Our data demonstrate that the E1 alpha and beta components of mammalian PDC are the M2'd' and 'e' mitochondrial autoantigens, respectively.