By light and electron microscopical examination it is shown that four structural components can contribute to obsolescent glomeruli: capillary basement membranes, enriched mesangium matrix, "vascular" hyalin and collagen fibers. Each of these components can bring about glomerular damage alone. One non-reactive form--a glomerular collapse with only basement membrane remnants--can be separated from three reactive forms: the accumulation of mesangium matrix (sclerosis or matrix-sclerosis), deposition of vascular hyalin (hyalinosis in the narrow sense), and fiber development within the former urinary space (fibrosis or fibro-sclerosis). The use of the term "fibrinoid" in place of the descriptive term "hyalin" is not supported by objective results. Knowledge of the various constituents which accumulate in the reactive types of glomerular obsolescence might be important in the diagnosis of the underlying disease, though mixed pictures were often observed. To avoid terminological overlap we suggest that the term "hyalinization" is replaced by "obsolescence" or "scarring" with specification of the structural components involved.