The composition of glomerulosclerosis was examined in focal sclerosis, membranoproliferative glomerulonephritis, and crescentic glomerulonephritis, using antisera specific for laminin and collagen types III and IV. Mesangial sclerosis consisted exclusively of extracellular matrix found in normal glomeruli, as did small synechiae in focal sclerosis and all synechiae in membranoproliferative glomerulonephritis. The large synechiae in focal sclerosis and all stages of the developing crescents associated with vasculitis contained mainly interstitial collagen, type III, a component not found in normal glomeruli. In the latter there were also disruptions of Bowman's capsule and an increase in the number of periglomerular interstitial cells. These observations suggested that glomerulosclerosis varied in composition and distribution, and some types of crescentic glomerulonephritis may have arisen from either cells resident in the glomerulus or cells coming from the interstitial space.