B lymphocytes committed to the production of IgG antibodies (Abs) to mitochondrial antigen such as pyruvate dehydrogenase complex(PDC) were quantitated in the peripheral blood of patients with primary biliary cirrhosis(PBC) using Epstein-Barr virus as a polyclonal activator of human B lymphocytes. B lymphocytes committed to the production of IgG Abs to PDC were found in high frequency in patients with PBC(0.54 +/- 0.16%, mean value +/- SE, of total IgG-producing B lymphocytes) in contrast to type C chronic hepatitis and healthy subjects in which they were less than 0.01%. The frequency of these B lymphocytes specific for PDC increased in parallel to the progression of the Scheuer's stage from I to II (stage I: 0.35 +/- 0.23%, stage II: 1.04 +/- 0.32%), but decreased with further progression to stage IV (stage III: 0.39 +/- 0.21%, stage IV: 0.07 +/- 0.06%). In addition, B lymphocytes specific for PDC decreased in the peripheral blood during the administration of cyclosporin A; this was accompanied by an improvement of lymphocyte infiltration severity in the liver. These data indicate that B lymphocytes specific for PDC are present in the peripheral blood of patients with PBC and their frequency reflects the degree of the lymphocyte infiltration in the liver.