Five patients with herpes gestationis, a blistering disease of pregnancy, were studied immunologically. All had in vivo deposition of C3 in a linear band along the basement membrane zone of lesional and normal-appearing skin, the location of early blister formation. Immunoglobulin deposition was more variable, though four patients had evidence of in vivo bound IgG at the same site. A circulating, complement binding herpes gestationis factor was demonstrated in the sera of four of the patients, its concentration unrelated to the activity of clinical disease. Characterization of this factor by sucrose gradient ultracentrifugation, specific absorption studies, and papain digestion indicates that it is an IgG. Evidence exists for involvement of both the classical and alternate complement pathways in vivo, though in vitro studies implicate the classical pathway as the primary route of complement activation. Three offspring were studied, none with clinical involvement; one showed in vivo deposition of C3 at the basement membrane zone of normal skin and a second showed the herpes gestationis factor in cord blood.