Thickening of the glomerular basement membrane (GBM) and expansion of the mesangial matrix are hallmarks of human diabetic nephropathy. Renal tissues from 15 patients with type II (non-insulin-dependent) diabetes (NIDDM) were studied by immunofiuorescence (IF) and immunogold electron microscopy (IEM) for the distribution of 2 type IV collagen peptides [α3(IV) noncoUagenous (NC) domain and α4(IV) NC domain] and 2 classical type IV collagen chains [αl(IV) NC domain and α2(IV) domain]. There was intense staining for α3(IV) NC and α4(IV) NC domain in the GBM but not in the mesangial matrix of patients with overt diabetic nephropathy. In contrast, staining with antibodies to αl(IV) NC and α2(IV) NC domain reacted with mesangial matrix but was significantly decreased in the GBM in the patients with overt diabetic nephropathy. IEM confirmed the IF findings. These data suggest that expansion of the mesangial matrix and thickening of GBM in NIDDM involves separate and distinct type IV collagen components and that the site-specific matrix alterations in NIDDM and type I (insulin-dependent) diabetes are parallel.