Several subtypes of anti-liver-kidney microsome antibodies (LKM) are known. LKM-1 antibodies associated with autoimmune chronic active hepatitis recognize P450 2D6, a cytochrome P450 mono-oxygenase. The frequent association of anti-LKM-1 antibodies and hepatitis C virus (HCV) infections and the probable existence of an infectious and autoimmune form of anti-LKM-1-associated hepatitis, requiring different therapeutical strategies, necessitates the exact determination of anti-LKM-1 specificities. Therefore, we compared various antibody tests (immunofluorescence, ELISA with recombinant P450 2D6, and Western blot with recombinant and natural antigens and agargel double diffusion) with sera of 27 anti-LKM-1-positive chronic active hepatitis (CAH) patients, with 61 sera harbouring anti-mitochondrial antibodies, 100 sera each from HCV-RNA-positive and HCV-RNA-negative patients, and 50 sera of healthy persons. Western blot techniques using recombinant MS2-polymerase P450 2D6 fusion protein were found to be the most sensitive and specific method for anti-LKM-1 antibody determination in routine laboratory. The recently recognized association of anti-LKM-1 antibody and HCV infection was confirmed by the results of this study. In anti-HCV and HCV-RNA-positive patients with anti-LKM-1 antibodies there was a preponderance of males with higher mean age and lower antibody titres. The results support the hypothesis of the existence of an autoimmune as well as an infectious (HCV triggered) subgroup of anti-LKM-1-positive hepatitis.