Using a monoclonal antibody (Mab-G3) recognizing glomerular basement membrane (GBM), we assayed GBM antigen (G3-Ag) in the urine and serum of renal disease patients by sandwich ELISA. The subjects included normal control (NOR), minimal change nephrotic syndrome (MCNS), IgA nephropathy (IgA), membranous nephropathy (MN), membranoproliferative glomerulonephritis (MPGN) and chronic renal failure (CRF). The urine and serum was used as the material. With urinary G3-Ag, there were no statistically significant differences among the NOR, MCNS, IgA, MN, MPGN and CRF groups. Although no correlation was observed with proteinuria, hematuria, serum creatinine, serum beta 2 microglobulin and urinary NAG, urinary G3-Ag showed a significant (p less than 0.05) increase in excretion in the group of progressive CRF patients with s-Cr more than 1.0 mg/dl/month as compared to the stationary CRF group with s-Cr less than 1.0 mg/dl/month. Serum G3-Ag showed lower values in almost all cases, and there were no significant differences among the renal disease groups. The above findings led us to believe that the assay of urinary G3-Ag was useful in determining the degree of GBM disorder. It was also presumed that assay of renal antigens in urine and serum with the respective anti-human renal monoclonal antibodies could be a new tool in diagnosing renal diseases.