In order to determine the source of epidermolysis bullosa acquisita (EBA) antigen, we studied its synthesis by human keratinocytes and fibroblasts in culture. To identify the antigen, we used the sera of 5 patients with EBA. These sera had antibodies directed against the epidermal basement membrane zone (BMZ) at titers of 5-80. The 5 sera were further characterized by immunoblotting on extracts of the epidermal BMZ. In concert with previous reports, 4 sera stained a 290 kD polypeptide, 3 sera weakly stained a 145 kD polypeptide, and 1 serum did not bind either polypeptide. To study the synthesis of the ERA antigen, cultured human keratinocytes and fibroblasts, derived from neonatal foreskins, were metabolically labeled with ¹⁴C-labeled amino acids. Radiolabeled newly synthesized proteins that were extracted from these cultures with nonionic detergent were used in an immunoprecipitation assay. The precipitated proteins were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. All 5 EBA sera, but none of 3 bullous pemphigoid or 4 normal human sera, precipitated a 290 kD polypeptide from extracts of both keratinocytes and fibroblasts. Approximately equal amounts of this 290 kD polypeptide were precipitated from equivalent amounts of extracts from either cell type. The 290 kD polypeptide was also specifically precipitated by ERA sera from extracts of a human squamous cell carcinoma line, SCC-15. The 145 kD polypeptide was not detected in the newly synthesized proteins of any of these cell cultures. This finding suggests that the 145 kD polypeptide is not a precursor of the 290 kD polypeptide. Taken together these results demonstrate that the EBA antigen is synthesized by both human keratinocytes and fibroblasts, and is not a tissue (epidermal)-specific product.