A group of children with autoimmune hepatitis is characterized by the presence in their sera of anti-liver-kidney microsome antibody (LKMA) as defined by immunofluorescence. Immunoblot analysis of the sera of 21 such children using rat-liver microsome total proteins as antigen allowed separation into three groups--group 1, whose sera recognized a 50 kDa protein; group 2, whose sera recognized a 66 kDa protein; and group 3, whose sera recognized both proteins. Patients with the anti-66-kDa reactivity more often displayed an acute onset of the disease, less signs of portal hypertension, better sensitivity to immunosuppressive therapy, and less tendency to relapse. They also displayed a lower titer of anti-rat-liver microsome antibody in enzyme-linked immunosorbent assay (ELISA) and a serum reactivity with a rat-liver cytosolic protein. These results (a) indicate that the LKMA-positive autoimmune hepatitis of children is heterogeneous from both clinical and immunological view points, (b) suggest that children with anti-66-kDa reactivity could have a less severe disease than children with the 50-kDa reactivity, and (c) indicate that immunoblot analysis should be added to ELISA and immunofluorescence studies to achieve better characterization of these patients.