Primary biliary cirrhosis is characterised by the presence of antimitochondrial antibodies which are directed against components of mitochondrial dehydrogenase complexes. The specificity of antimitochondrial antibodies for primary biliary cirrhosis as detected by immunoblotting was investigated. Commercially available preparations of pyruvate and oxo-glutarate dehydrogenases and beef-heart mitochondria were used as source of antigens. Sera from 47 primary biliary cirrhosis patients (46 of whom were antimitochondrial antibody positive by immunofluorescence), 16 non-primary biliary cirrhosis patients (antimitochondrial antibody positive by immunofluorescence), 23 liver-kidney microsomal antibody positive chronic active hepatitis patients, and 32 patients with connective tissue diseases were examined. Of the 47 subjects with primary biliary cirrhosis, 43 (91%) and 13 (28%) tested positive by immunoblotting for pyruvate and oxo-glutarate dehydrogenase, respectively. Only three primary biliary cirrhosis patients were negative for both antigens, including the only one shown to be antimitochondrial antibody negative by immunofluorescence. The other two patients were positive by immunoblotting with beef-heart mitochondria. In contrast, only three of 16 (19%) non-primary biliary cirrhosis patients who were antimitochondrial antibody positive by immunofluorescence tested positive by immunoblotting (for both pyruvate dehydrogenase and beef-heart mitochondria). None of the 23 liver-kidney microsomal antibody positive and the 32 patients with rheumatic diseases were positive by immunoblotting with any antigen. Our data show that immunoblotting with commercially available oxo-acid dehydrogenases is a reproducible method for the detection of antimitochondrial antibodies highly specific for primary biliary cirrhosis.