The autoantibodies found in the sera of patients with primary biliary cirrhosis have been shown to react with a component of the mitochondrial inner membranes. Outer membranes were inactive. The purity of the inner and outer membrane fractions obtained by 2 different methods was assessed by electron microscopy and marker enzyme tests. Using negative-staining it was possible to visualize antibody binding to mitochondrial membranes. At high resolution it could be seen that the 90° particles on the cristal membranes were not involved in the reaction with antibody, but it was not possible to establish in the present studies the precise antigenic site upon the mitochondrial inner membranes.