IgG, IgA and IgM class antibodies to mouse laminin and human fibronectin in sera from patients with various glomerular diseases (50 cases of IgA nephropathy, 5 cases of minimal-change nephrotic syndrome; 6 cases of membranous nephropathy, 5 cases of systemic lupus erythematosus, 2 cases of Henoch-Schönlein purpura, 3 cases of poststreptococcal nephritis and 4 cases of preeclampsia) and from 30 normal controls were tested using a solid-phase enzyme-linked immunosorbent assay method. IgA antimouse laminin antibody titers in sera from IgA nephropathy patients were significantly higher (p less than 0.05) than in controls. There were no statistical differences in IgA antimouse laminin antibody titers between patients with other glomerular diseases and normal controls. IgM antimouse laminin antibody was significantly raised (p less than 0.01) in sera from patients with preeclampsia. The reaction of mouse laminin with the IgA nephropathy and preeclampsia sera on each of the IgA and IgM assay systems was inhibited by the antigen at up to 5 micrograms/ml. However, it was not inhibited by anti-C3d, anti-C1q, anti-J chain and antisecretory component sera or saccharides. The reaction of mouse laminin with an exceptionally high-titer IgA antimouse laminin antibody serum from a normal control on the IgA assay system was clearly inhibited by 1 mM of melibiose, which contains alpha-galactosyl residues. The same concentration of melibiose, however, did not inhibit the reaction of mouse laminin with IgA nephropathy sera on the same assay system. Treatment of mouse laminin with alpha-galactosidase did not alter any binding from IgA nephropathy sera but binding was lost from an exceptionally high-titer normal control serum. There were no correlations between serum IgA level and IgA antimouse laminin antibody titer in sera from IgA nephropathy patients. Immunoblot techniques revealed the presence of antibody in sera from IgA nephropathy patients reacting with both subunits A and B of laminin, somewhat stronger with laminin A. None of the sera tested contained antifibronectin antibodies. These results indicate that the IgA antimouse laminin antibody is a specific antibody in IgA nephropathy and might play a role in the pathogenesis of the nephritis since mouse laminin and human mesangial laminin present a common epitope.