The serum of a patient with a typical case of pemphigus vulgaria contained complement-fixing intercellular autoantibodies (pemphigus antibodies). Complement-fixing pemphigus antibodies were demonstrated only during the untreated active stage of the disease and titrated lower than corresponding IgG autoantibodies, which paralleled well with the disease activity. Pemphigus lesional skin, which contained in-vivo-bound IgG, showed the capability of further binding C3 in vitro from normal human serum. It was suggested from these findings that the complement system may play an active role in pemphigus acantholysis through complement-fixing pemphigus antibodies.