Bullous pemphigoid (BP) and herpes gestationis (HG) are subepidermal blistering diseases associated with an autoimmune response directed against BP180, an epidermal hemidesmosomal glycoprotein. The pathogenic relevance of this Ag/Ab system was established by the recent demonstration that IgG Abs reactive with the murine form of BP180 (mBP180) are capable of triggering a subepidermal blistering disease after passive transfer into neonatal BALB/c mice. The aim of the present study was to determine the fine specificity of the pathogenically relevant Abs in this experimental model of BP. Four high titer rabbit-anti-mBP180 antisera were included in this analysis--only two of which exhibited pathogenic activity in the passive transfer model. Immunoblot analysis using a panel of mBP180 deletion mutants revealed that each of the four rabbit sera reacted with at least three distinct sites on the mBP180 ectodomain; however, this technique failed to distinguish between the reactivity patterns of the pathogenic and nonpathogenic sera. An alternative technique, liquid phase immunoadsorption analysis, was used to identify one mBP180 antigenic site, comprising 9 to 12 amino acids and designated mBP1, that was specifically recognized by the two pathogenic sera. Pre-adsorption of pathogenically active IgG preparations with fusion proteins containing the mBP1 antigenic site resulted in complete blocking of immunofluorescence reactivity with the murine basement membrane zone (BMZ) and in complete neutralization of pathogenic activity. Anti-BMZ reactivity displayed by nonpathogenic Abs was not altered or diminished by pre-adsorption with this same mBP180 recombinant protein. These findings should help to elucidate the immunopathologic mechanisms responsible for human BP and HG and may have significant implications in the diagnosis and treatment of these autoimmune diseases.