The antigen involved in the glomerulonephritis associated with antibodies to glomerular basement membrane (GBM) was purified from human GBM digested with highly purified clostridial collagenase. The purified nonreduced sample contained two components with closely similar mobilities on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. After reduction they moved as one, nonantigenic, component, corresponding to a molecular weight of 26,000. Immunologically identical aggregates of higher molecular weight (i.e., 48,000) were also identified in the crude digest. Reduction of such aggregates after purification released some protein with a molecular weight of 26,000, but a large proportion was insensitive to reduction. Seven patients with Goodpasture syndrome all had circulating anti-GBM antibodies directed only against the purified antigen.