Anti-liver-kidney microsome antibody type 1 (LKM1), present in the sera of a group of children with autoimmune hepatitis, was recently shown to recognize a 50 kDa protein identified as rat liver cytochromes P450 db1 and db2. High homology between these two members of the rat P450 IID subfamily and human P450 db1 suggested that anti-LKM1 antibody is directed against this human protein. To test this hypothesis, a human liver cDNA expression library in phage lambda GT-11 was screened using rat P450 db1 cDNA as a probe. Two human cDNA clones were found to be identical to human P450 db1 by restriction mapping. Immunoblot analysis using as antigen, the purified fusion protein from one of the human cDNA clones showed that only anti-LKM1 with anti-50 kDa reactivity recognized the fusion protein. This fusion protein was further used to develop an ELISA test that was shown to be specific for sera of children with this disease. These results: 1) identify the human liver antigen recognized by anti-LKM1 auto-antibodies as cytochrome P450 db1, 2) allow to speculate that mutation on the human P450 db1 gene could alter its expression in the hepatocyte and make it auto-antigenic, 3) provide a simple and specific diagnostic test for this disease.