To study the subclass distribution of autoantibodies and their complement-fixing capacity in cicatricial pemphigoid (CP) and epidermolysis bullosa acquisita (EBA) we studied the sera from 23 patients by both indirect immunofluorescence (IIF) on 4-microns cryostat sections of normal human skin and immunoblotting of epidermal or dermal extracts. Monoclonal antibodies of strict specificity for human IgG subclasses were used. Sera from 20 patients with BP served as controls. In addition, total IgG subclass levels were determined by indirect competitive ELISA in all sera. Complement binding capacity was studied by IIF using antibodies to C3 after incubation of skin section with autoantibodies and source of fresh complement. CP autoantibodies reacting with the 230-240 kD and/or the 180-kD epidermal bands showed an IgG4/IgG1 subclass restriction, with a predominance of IgG4 in 10 cases, of IgG1 in four. In BP sera, IgG4 and IgG1 autoantibodies were detected with a similar frequency (100% and 83%, respectively). In EBA sera, autoantibodies reacting with the 290 kD and 145 kD dermal bands also showed an IgG1/IgG4 restriction. Concordant results were obtained by IIF. However, the IIF method had a lower sensitivity for the detection of IgG4 CP antibodies and IgG1 EBA antibodies than immunoblotting. Finally, when CP antibodies were analyzed for their complement-binding activity, it was found that sera containing IgG4 autoantibodies alone never fixed complement whereas all complement-fixing CP sera had IgG1 autoantibodies, suggesting that only this subclass of antibodies is capable of fixing complement.