Goodpasture's disease, an autoimmune disorder causing severe glomerulonephritis and pulmonary haemorrhage, is characterized by antibodies to the glomerular basement membrane (GBM). The principal target antigen has been identified as the carboxyl terminal non-collagenous (NC1) domain of the alpha3-chain of type IV collagen. Anti-GBM antibodies appear to recognize one major epitope that is common to all patients, and is largely conformational. We have analysed antibody binding to recombinant alpha(IV)NC1 domains using a construct and expression system shown to produce correctly folded antigen that is strongly recognized by autoantibodies. In this system, as with the native antigen, alpha3(IV)NC1 was bound strongly by antibodies from all patients, whereas the closely related alpha1(IV) and alpha5(IV)NC1 domains, similarly expressed, showed no such binding. A series of chimeric NC1 domains, between human alpha3(IV) and alpha1(IV), and between human and rat alpha3(IV), were expressed as recombinant molecules, and were recognized by autoantibodies to varying degrees. Strong binding required the presence of human alpha3(IV) sequence in the amino terminal region of both sets of chimeric molecules. This work strongly suggests that the amino terminal of alpha3(IV)NC1 is critical for antibody recognition, whereas the carboxyl terminal end of alpha3(IV)NC1 has a less important role.