The immunohistochemical localization of the extracellular matrix was examined in 31 cases with different degrees of human diabetic nephropathy using antisera to human collagen types I, III, IV, V, fibronectin, laminin, and basement-membrane-associated heparan sulfate proteoglycan (HSPG). In normal glomeruli, HSPG was predominantly localized in the glomerular basement membrane and in the mesangium, and to minor extent in the basement membranes of tubules and Bowman's capsule. Collagen IV and laminin were distributed in glomerular basement membrane and mesangium in minor amounts. Interstitial collagens usually do not occur within glomeruli except for collagen V which has a light microscopic glomerular distribution similar to collagen IV. In diabetic diffuse glomerulosclerosis, the enlarged mesangial matrix showed an increased staining reaction for collagen IV, V, laminin, and fibronectin whereas the staining pattern of HSPG was markedly reduced. Early, small nodular lesions in diabetic glomeruli were similarly positive for most of the basement membrane components, whereas HSPG remained absent. With an increase in the diameter of the noduli, however, the staining reaction for all basement membrane components diminished, whereas interstitial collagens V and III, but not collagen I, were present in these noduli in substantial amounts. These initial studies provide evidence that the changes in the glomerular matrix in diabetic nephropathy may be divided into distinct and progressing stages of lesions. The reduced amount of HSPG even in slight, early lesions may represent the morphologic correlate to the impaired filter function of the glomerular basement membrane.