An antigen expressed at the dermal-epidermal junction as well as in some other human basement membranes (BM) has been detected by the use of a monoclonal antibody termed GB3. This antigen, synthesized by cultured normal human keratinocytes, has been identified as a 600-kilodalton glycoprotein different from other known components of BM. Using indirect immunofluorescence, GB3 was found to be not reactive with the epidermal BM in patients with lethal junctional epidermolysis bullosa. The present study demonstrates (by indirect immunofluorescence) that GB3 defines a widespread defect of several BM in these patients. Furthermore, it gives evidences for an intrinsic biologic defect of lethal junctional epidermolysis bullosa epidermal keratinocytes using in vitro culture of these cells. Whether the lack of GB3 reactivity is the consequence of a true absence of the antigen or an alteration of its molecular structure is not yet known. Nevertheless, GB3 is a useful probe for both rapid and prenatal diagnosis of lethal junctional epidermolysis bullosa, which will give new insights into the molecular comprehension of this disorder.