Antibodies present in the sera of a group of children with autoimmune hepatitis react with human cytochrome P450 IID6. cDNA constructions of various fragments of human P450 IID6 were made and expressed and the resulting peptides were tested in immunoblot with patients' sera. These allowed identification of at least two antigenic sites on the P450 molecule. The main one, recognized by all sera tested, is located between amino acids 239 and 271. Synthesis of three peptides covering this area of the molecule allowed identification of a sequence of three amino acids (tyrosine-tryptophane-asparagine) located at position 261-263 that constitutes the essential part of the epitope. A protein sequence data-base search revealed homologies between this region of human P450 and proteins from Salmonella typhimurium, from human T lymphotropic virus types 1 and 2 and Herpes simplex virus type 1.