Using one-dimensional and two-dimensional immunoblotting techniques the reactions of sera from 14 patients with liver-kidney microsomal (LKM) antibody positive chronic active hepatitis (CAH) with human liver microsomal preparations was compared with the reaction of sera from 12 healthy persons and from five patients with systemic lupus erythematosus (SLE). All sera displayed a multiplicity of reactions. This demonstrates the presence of many autoantibodies in normal human sera. It could be shown that all sera react with the cytoskeletal antigens cytokeratin, actin and actomyosin. These reactions were more marked in the autoimmune sera, i.e. LKM-positive CAH and SLE. Densitometric subtraction was found to be a reliable technique to distinguish the natural antigen/autoantibody reactions from pathological, disease-characteristic autoantibodies. It was shown that the pathological LKM autoantibodies recognize non species-specific microsomal proteins at 50 kD of pI 7.5-8.0 at high titres, which are only very weakly recognized by normal or SLE sera. We recommend sensitive immunoblotting techniques and densitometric subtraction as the currently most accurate method to distinguish natural from pathological autoantibodies.