Abstract:A fluid phase radioimmunoassay (RIA) has been established for the detection of circulating anti‐glomerular basement membrane (GBM) antibodies. Antibodies were detected by binding with a collagenase solubilised, immunochemically purified, radiolabeled extract of human particulate GBM. Sera from 108 patients with a variety of histological types of glomerulonephritis (GN) were assessed for the presence of antibodies. Likely positive sera were those from patients with clinical features of rapidly progressive glomerulonephritis (RPGN) with crescentic or diffusely proliferative GN and linear staining of their GBM's with IgG. Thirty one of 35 sera from such patients were positive in the RIA. Sera from 14 patients with RPGN but with granular IgG deposition (i.e. immune complex related GN) were all negative for anti‐GBM antibodies as were sera from 59 patients with a variety of non‐crescentic non RPGN without linear IgG. Comparison of the RIA with conventional indirect immunofluorescence (IIF) showed that the RIA had a higher detection rate of antibodies in likely positive sera (19/21 RIA: 16/21 IIF) lower false positive detection rate with sera from patients without GN (0/26 RIA: 2/26 IIF) and was three to four times more sensitive in serial dilution st