Abstract Using in vitro complement immunofluorescent staining methods, serum samples from 5 active cases of bullous pemphigoid, with pemphigoid antibody titers of 320 or greater, were tested for their ability to fix the regulatory protein β1H globulin in addition to C4 and C3. All 5 samples yielded positive C3, C4 and β1H staining reactions in a linear fashion along the basement membrane zone. Heat inactivation or treatment of the complement source (fresh normal human serum) with EDTA, Mg2 EGTA abolished all 3 staining reactions. Substitution of C2-deficient serum as the source of complement inhibited both C3 and β1H staining but had no effect on C4 staining. Use of serum devoid of β1H (Rβ1H) minimally enhanced C3 staining while no β1H staining was observed. The addition of β1H to RΒ1H restored positive β1H staining. Skin biopsies of perilesional skin from 6 patients with bullous pemphigoid demonstrated heavy in vivo deposition of β1H in addition to C3. These studies suggest that pemphigoid antibodies will fix the regulatory protein β1H in addition to other complement components, a phenomenon which requires activation of the classical complement pathway and generation of the C3b amplification convertase.