An indirect immunofluorescence technique was used to investigate the immunoglobulin class, IgG subclass, light chain type and complement fixing ability of anti-liver kidney microsomal antibody (anti-LKM) in the sera of six children and five adults with autoimmune chronic active hepatitis (aCAH), Anti-LKM titres ranged from 1/80 to 20,480. In the children, the antibody belonged to the IgG1 subclass alone (titre l/80-1/20,480) and was able to fix complement (titre 1/40 -1/5120). In the adult group, antibody belonged to the IgG1 subclass in three cases (titre 1 40-1 640 whilst two belonged to both IgG1 (titre 1/640) and lgG4 (titre 1/40, 1/640). Such subclass restriction is similar to that found in other autoimmune disorders and may be genetically determined. Investigation of' the light chain constituent of anti-LKM revealed that the kappa to lambda ratio was consistent with a polyclonal antibody response. To investigate the nature of the antigen to which anti-LKM is directed, the ability of these sera to bind to the surface membrane of isolated human hepatoma cells (Alexander cells) was investigated. Four of the eleven sera showed significant binding activity. Prior incubation of these four sera with Alexander cells abolished their membrane binding activity, but did not alter the anti-LKM titre. These results suggest that anti-LKM binds to cytoplasmic constituents alone and does not cross-react with surface antigens. [ABSTRACT FROM AUTHOR]