1. The sera of more than 90% of patients with primary biliary cirrhosis contain antimitochondrial antibodies which react with the E2 component of the pyruvate dehydrogenase complex, identified as the major autoantigen in primary biliary cirrhosis. All previous studies in this area have utilized protein derived from animal tissue or have used recombinant human pyruvate dehydrogenase complex E2 expressed in Escherichia coli. 2. We report the preparation and characterization of native pyruvate dehydrogenase complex and pyruvate dehydrogenase complex E2 from human heart tissue and its application in studies of immune reactivity with the sera of patients with primary biliary cirrhosis. 3. The immune reactivity of sera from patients with primary biliary cirrhosis versus the bovine and human E2/X components of pyruvate dehydrogenase complex was indistinguishable in both immunoblotting and the more sensitive e.l.i.s.a. 4. These findings suggest that the reactivity of sera from patients with primary biliary cirrhosis against the major autoantigen of the disease is a property of that antigen, independent of its human or bovine origin. Furthermore, this justifies the use of bovine pyruvate dehydrogenase complex in past and future work on primary biliary cirrhosis antibody reactivity.