Bullous pemphigoid (BP) is an autoimmune blistering disease characterized in part by the presence of tissue-bound and circulating antibodies (mostly of IgG) to the basement membrane zone (BMZ). We previously reported that IgG subclasses of BP antibodies were IgG1, IgG2and IgG4, and that only BP IgG1fixed complements. In this study, we examined whether BP IgG sub-classes bound to the same epitope of BP antigen or a different epitope. In an inhibition immunofluorescence studies, the complement fixing capability of IgG1was inhibited by the pretreatment with IgG4and partially inhibited by IgG2. On immunoblot analysis, IgG1and IgG4were bound to the same MW of BP antigen. In enzyme-linked immunosorbent assay (ELISA), the binding capability of IgG subclass fractions from patients with BP to synthetic peptide PI-2, exceeding normal IgG subclass fractions was seen in five IgG1, one IgG2and two IgG4, from eight BP patients. The binding capability of IgG subclass fractions from the patients with BP to Pl-1, exceeding the normal IgG fractions was seen in two IgG1, three IgG2and one IgG4from ten BP patients. On inhibition ELISA, the binding activity to Pl-2 of IgG4was partially inhibited by the pretreatment of IgG1and IgG2. These findings suggest that BP IgG1, IgG2and IgG4could bind to the same epitope though considerable variation occurred between patients.