Sera from patients with primary biliary cirrhosis inhibit the activity of the mitochondrial pyruvate dehydrogenase complex. We utilized this effect to develop a simple, miniaturized, semiautomated spectrophotometric assay as a diagnostic aid. The sera studied were from 71 patients with primary biliary cirrhosis and 62 other subjects. The assays included enzyme inhibition, immunofluorescence on HEp-2 cells, enzyme-linked immunosorbent assay using recombinant pyruvate dehydrogenase complex-E2 and immunoblotting on bovine heart mitochondria. With the 71 primary biliary cirrhosis sera, on which M2 antibody was detected by immunofluorescence in 64 (90%), antibodies against pyruvate dehydrogenase complex were detected in 53 (83%) by means of enzyme inhibition, in 57 (89%) by means of enzyme-linked immunosorbent assay and in 60 (94%) by means of immunoblotting. Of the 64 sera positive by immunofluorescence, 60 reacted with pyruvate dehydrogenase complex-E2 on immunoblotting, and the miniaturized enzyme inhibition assay was positive in 53 of these. The enzyme inhibition assay and enzyme-linked immunosorbent assay were calibrated to give a specificity of 100%. At this level, the sensitivities for detection of pyruvate dehydrogenase complex antibody were 83% and 87%, respectively. We found no significant changes in levels of reactivity with the enzyme inhibition assay or enzyme-linked immunosorbent assay according to disease stage. Treatment with cyclosporine was accompanied by a significant decrease in levels of antibody to pyruvate dehydrogenase complex-E2 that matched improved indexes of biochemical liver function. The semiautomated enzyme inhibition assay as described provides an additional diagnostic procedure in pyruvate dehydrogenase complex; this type of assay involving specific enzyme inhibition could also be generically applied to any autoantibody antigen system. (Hepatology 1994;20:1220–1224).