IgG autoantibodies with specificity for glomerular antigenic determinants have been detected in the sera of 23 of 75 patients with IgA nephropathy. The antigen or antigens are distinct from the Goodpasture antigen and have at least three components that bind IgG. The components are of molecular weight 185, 160, and 30-40 kD by high performance liquid chromatography, and, deaggregated, 69 to 30 kD by immunoblotting, with variation in affinity between patients. The autoantibody was detected more frequently in patients with IgG glomerular deposits, was present in sera intermittently, and was associated with clinical evidence of nephritis. IgG autoantibody secretion may be a component of the polyclonal B cell activation that accompanies infection-induced nephritis in IgA nephropathy.