This study characterizes a novel basement membrane component that is the target of autoantibodies in patients with linear IgA bullous dermatosis. Tissue surveys showed that this protein localized to the epidermal side of 1 M NaCl split skin and to basement membranes in cornea, oral mucosa, esophagus, intestine, kidney collecting ducts, ureter, bladder, urethra, and thymus, but was absent in lung, blood vessels, skeletal muscle, and nerve. Monoclonal antibody 123, which recognizes this protein, induced dermal-epidermal separation of human skin in situ, and this protein was found, by immunoelectron microscopy, to localize exclusively to anchoring filaments. This protein was secreted as as a 120-kDa peptide from primary cultures of keratinocytes as determined by radioimmunoprecipitation. Monoclonal antibody 123 recognized this protein as a 120-kDa band from conditioned cell culture medium and a 97-kDa band from human skin extracts as shown by immunoblot. Serum from five patients with the autoimmune blistering disorder linear IgA bullous dermatosis specifically recognized bands of 120 and 97 kDa from culture medium and skin extracts, respectively, that were of identical electrophoretic migration to the bands recognized by monoclonal antibody 123. In summary, this study characterizes a novel anchoring filament protein that is the target of linear IgA bullous dermatosis autoantibodies. Because monoclonal antibody 123 induces blistering of human skin, we hypothesize that this protein functions to maintain dermal-epidermal cohesion and that autoantibodies in this disease are themselves pathogenic. We propose LAD-1 as the name for this protein.