Some patients with hereditary nephritis (HN) who have received a renal transplant have been shown to form antibody with specificity for the NC1 domain of collagen type IV, a major constituent of glomerular basement membranes (GBM). We attempted to duplicate this phenomenon in a family of dogs with X-linked HN, a model for human X-linked HN, by immunizing affected male dogs with normal dog NC1 domain. A collagenase digest was prepared from normal dog GBM, the NC1 domain was separated into dimer (approximately 50 kDa) and monomer (24 kDa and 26 kDa) components by SDS-PAGE, and injected into two affected male dogs. Antisera obtained from both dogs contained antibody which reacted with the NC1 domain of dog and human GBM by a plate-binding radioimmunoassay, bound to the dimer and 26 kDa monomer bands by Western blotting, and staining dog and human GBM by immunofluorescence (IF). The affected male dog antiserum reacted equally by radioimmunoassay with the NC1 domain isolated from GBM of unaffected, affected male, and carrier female dogs in the family with X-linked HN, and bound by Western blotting to dimers and the 26 kDa monomer band of the NC1 domain of GBM in each group of dogs. However, the affected male dog antiserum differentiated these dogs by IF; it produced global staining of GBM of unaffected dogs, failed to stain GBM of affected male dogs, and produced segmental staining of GBM of carrier female dogs. Absorption of the affected male dog antiserum with normal dog NC1 domain eliminated the staining of dog GBM by IF, whereas staining persisted after absorption with affected male dog NC1 domain. The abnormal staining patterns of GBM seen by IF in the affected male and carrier female dogs and the results of the absorption studies imply an abnormality of one or more determinants in the 26 kDa monomer band of the NC1 domain of their GBM. Amino acid sequencing of this band identified the alpha 1(IV) chain of collagen type IV, a finding that has implications for the pathogenesis of canine X-linked HN. Absent and segmental staining respectively were also seen by IF in GBM of a male and female patient with HN, using the affected male dog antiserum. Thus, the results obtained in affected male and carrier female dogs with X-linked HN may also be relevant to patients with this disease.