X-linked Alport syndrome (AS) is a heritable disorder which is associated with mutations in the type IV collagen alpha 5 (IV) chain gene (COL4A5) located on chromosome X. Following renal transplantation, an average of 6% of male AS patients develop anti-GBM nephritis. We studied the specificity of the antibodies against type IV collagen in the serum of a patient with COL4A5 partial deletion. The specificity of these alloantibodies was determined against collagenase-digested GBM, as well as against recombinant non-collagenous (NC1) domains of the type IV collagen alpha 1(IV)-alpha 6(IV) chains expressed in escherichia coli. Immunoblotting and ELISA demonstrated that these antibodies bound specifically to the NC1 domain of alpha 5(IV) collagen. There was no binding to the NC1 domain of the other chains, including the Goodpasture antigen. Competitive ELISA confirmed the results obtained by ELISA and immunoblotting. This patient developed alloantibodies directed against antigens present in the grafted kidney, but absent from his Alport kidney. The pathogenesis of post-transplantation glomerulonephritis in the Alport patient studied is thus similar to that of Goodpasture syndrome, with the exception that the pathogenic antibodies are targeted to another alpha chain of type IV collagen.