Bullous pemphigoid (BP) antibodies are known to react with an antigen of the basement membrane zone (BMZ) of squamous epithelia and produce, by the indirect immunofluorescence technique, linear fluorescence at the BMZ. Direct and indirect immunoelectron microscopy (IEM) have demonstrated BP antigen to be within the lamina lucida, in close association with the basal cell membrane. Trypsin-dissociated epidermal basal cells bind BP antibodies in a polar distribution, presumably because the BP antigen is restricted to the dermal pole of the basal cell membrane. In this study we have utilized newborn BALB/c mouse skin to obtain both dissociated basal cells (by trypsinization) and epiderinal sheets (by dithiothreitol treatment). We show that viable basal cells, which are impermeable to IgG molecules, do not react with HP antibodies. When the basal cell plasma membrane is disrupted by cytospin centrifugation, air drying, freezing and thawing, or hypotonic lysis, or permeated by non-ionic detergents (saponin), cells become reactive with BP antibodies. Basal cell cytoskeletons, prepared by sequential treatment with Triton X-100, deoxyribonuclease and 2 M NaCl continue to react with BP antibodies. Similarly, viable epidermal sheets fail to bind BP antibodies. When epidermal sheets are treated with non-ionic detergents, water, or freezing and thawing prior to incubation with BP antibodies, linear BMZ fluorescence is observed. IEM study of saponin-treated basal cells shows the immunoreactants to be localized on intracytoplasmic vacuoles which represent internalized hemidesmosomes. IEM of permeated epidermal sheets shows the immunoreactants as aggregates on the inner surface of the dermal pole of the basal cell membrane. These observations suggest that the BP antigen is intracellular and is in close association with the basal cell cytoskeleton and hemidesmosomes.