Linear IgA bullous dermatosis (LABD) is a rare blistering skin disease characterized by basement membrane zone deposition of IgA. This study identifies a tissue antigen detected by patient serum and then isolates the autoantibody using epidermis and protein bands blotted on nitrocellulose as immunoabsorbents. Sera from 10 patients (9 with cutaneous disease and 1 with cicatrizing conjunctivitis) were evaluated. Indirect immunofluorescence revealed an IgA anti-basement membrane antibody in 6 of 10 sera with monkey esophagus substrate and 9 of 10 sera with human epidermal substrate. Immunoblotting was performed on epidermal and dermal extracts prepared from skin separated at the basement membrane zone with either sodium chloride or EDTA. Saline-separated skin expressed a 97-kD band in dermal extract alone that was recognized by 4 of 10 sera. EDTA-separated skin expressed the 97-kD band in both epidermal (4 of 10 sera) and dermal (6 of 10 sera) extract. Immunoabsorption of positive sera with epidermis purified an IgA antibody that reacted uniquely with the 97-kD band. In addition, IgA antibody bound to nitrocellulose was eluted from the 97-kD band and found to uniquely bind basement membrane zone. It is likely that the 97-kD protein identified by these techniques is responsible for basement membrane binding of IgA in LABD.