Recently we described a monoclonal antibody, designated 7-5Q, with specificity for a human non-glomerular basement membrane capillary wall antigen. In order to purify and characterize the corresponding antigen for the development of sensitive ELISA techniques, applicable to the diagnosis and monitoring of renal disease, human kidney cortices were extracted with a variety of detergents. A double band of 98/105 kD was evident by immunoblotting in all preparations most notably with a Triton X-114 extract. The caprylic acid- and ammonium sulfate-purified monoclonal antibody 7-5Q was covalently bound to CNBr-activated Sepharose and detergent extracts were applied to this affinity material. A pure protein of 98/105 kD (double band on SDS-polyacrylamide gels) was eluted. Glycan typing with lectins revealed N-acetyl glucosamine-residues and amino acid analysis a relatively high content of acidic (31%) and hydrophobic (30%) amino acids, indicating that the antigen is an acidic, membrane-bound glycoprotein.