Our previous studies showed that 54 kD and 48 kD tubular basement membrane (TBM) proteins were the major form of the target antigen involved in anti-TBM antibody-mediated tubulo-interstitial nephritis in humans. In those studies, we isolated the 54 kD glycoprotein (named gp54) from collagenase-digested bovine TBM. NH2-terminal amino acid sequencing indicated that gp54 represented a newly defined glycoprotein. In this study, we further characterized the target antigen, using mouse monoclonal antibodies to gp54 and polyclonal anti-gp54 peptide antibody. Two monoclonal antibodies (H79 and H80) were established, and they reacted, by immunofluorescence, predominantly with the proximal TBM of humans, rabbits, and Wistar, Sprague-Dawley, and Brown-Norway rats, but not with that of Lewis rats. They were also fixed by blotting intensely to the 54 kD component and weakly to the 48 kD component of collagenase-digested human TBM.In vivotransfer of H79 to Wistar rats showed extensive linear binding of mouse IgG to the TBM and the basal membrane of the small intestine; however, no pathologic changes were seen by light microscopy. The anti-gp54 peptide antibody reacted with both the 54 kD and 48 kD TBM components of human TBM. mRNA was prepared from rabbit kidneys, and fractionated to enrich mRNA encoding the 54 kD and 48 kD peptides. Onin vitrotranslation experiments with the mRNA fraction, the 54 kD and 48 kD peptides were immunoprecipitated with anti-gp54 antibodies. These findings indicate that the 54 kD and 48 kD components are encoded with different mRNA, but that they share the same antigenic epitope. It is likely that they are structurally related and exist as isoforms.