The GBM is an important structure for the development of immunologic renal disease. It may serve as the primary antigen or as the site of deposition of immune complexes. The GBM is recognized as the major protein filtration barrier of the glomerulus. The composition of the GBM includes collagen, predominantly but not exclusively type IV, which is present as procollagen. Two glycoproteins, laminin and fibronectin, are either intrinsic components of the GBM or intimately associated with the membrane. Heparan sulfate, a glycosaminoglycan, is contained within the substance of the GBM and seems to comprise the primary anion of the structure. Antibodies to these individual GBM components are being recognized with increasing frequency in patients with glomerulonephritis. The recognition of these antibodies may serve to expand our definition of both immunologic and degenerative diseases of basement membrane.