The possible involvement of basement membrane-associated collagen (recognized by the monoclonal antibody JK-132) in the evolution of diabetic nephropathy was studied in kidney specimens from seven patients with noninsulin-dependent diabetes mellitus, and its distribution was compared with those of antibodies against alpha 1 to alpha 4 chains of type IV collagen. JK-132, a monoclonal antibody against basement membrane-associated collagen, reacted immunohistochemically exclusively with the mesangial matrix of the glomerular capillary. In contrast, antibodies to the alpha 1 and alpha 2 chains (IV) reacted strongly with mesangial matrix, and less strongly with the glomerular basement membrane (GBM). Antibodies to the alpha 3 and alpha 4 chains (IV) reacted mainly with GBM. In diabetes, JK-132 reacted most extensively with the expanded mesangial matrix, its staining intensity increasing with progression of the diabetic glomerulosclerosis. Antibodies to the alpha 1 and alpha 2 chains (IV) reacted prominently with the expanded mesangial matrix but less strongly with the GBM. Antibodies to the alpha 3 and alpha 4 chains reacted intensely with the thickened GBM. These results suggest that basement membrane-associated collagen differs from alpha 1 to alpha 4 chains of type IV collagen and that basement membrane-associated collagen is a good marker of mesangial expansion in diabetic nephropathy.