We describe a sensitive radio-immunoassay for auto-antibodies to glomerular basement membrane (GBM). The assay discriminates patients with glomerulonephritis associated with circulating auto-antibodies to glomerular basement membrane from those with other forms of nephritis. Monitoring of antibody levels in sequential serial serum samples enables drug therapy to be tailored precisely to control the aberrant immune response, a situation unique in the management of glomerulonephritis.