Monoclonal antibodies were prepared in an attempt to develop diagnostic tools for the identification of toxic damage to the renal papilla. One of these antibodies, termed PAP X 5C10, recognizes an antigen that is located on the luminal side of collecting duct epithelial cells. As shown by Western blotting experiments, this antigen can be extracted from papillary tissue and visualized as a broad band of high molecular weight. Enzyme-linked immunosorbent assays have shown that increased amounts of this antigen can be detected in urine of rats treated with bromoethaneamine. This procedure thus enables this antigen to be detected also in supernatants of cultured ductal cells.