The nephritogenic brush border glycoprotein antigen (gp600) of autoimmune membranous glomerulonephropathy (Heymann nephritis) isolated from crude proximal renal tubular membrane fraction (FX1A) by Lens culinaris lectin affinity chromatography was characterized. The molecular weight of the antigen determined by molecular exclusion chromatography was found to be 600,000. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis it resolved into five polypeptides which were in the molecular weight range of 70,000 to 330,000. All five polypeptide bands stained for carbohydrates and lipids with Schiff's reagent and Sudan black, respectively. Rabbits immunized with gp600 produced an antibody that specifically reacted with the brush border of proximal renal tubules of rat by indirect immunofluorescence and gave a single precipitin line in double diffusion in gel (Ouchterlony) against the antigen. The specificity of gp600 was proven by (a) the ability of gp600 to produce active Heymann nephritis, (b) the ability of anti-gp600 to produce passive Heymann nephritis, and (c) the ability of gp600 alone, out of other antigens in FX1A, to specifically react with the nephritogenic autoantibody. The latter was tested by an enzyme immunodiffusion technique described in this report.