AbstractThe distribution of type I collagen in normal and diseased renal tissues was studied using immunofluorescence and immunoelectron microscopy, and was compared with that of type III collagen. In normal human kidneys, a monoclonal antibody against type I or type III collagen reacted with the renal interstitium, but not with the intra‐glomerular structures. In various types of glomerulonephritis, immunofluorescent staining for type I collagen was positive in the fibrocellular and fibrous crescents, sclerosed glomeruli, and infrequently within the glomerular mesangium. In the crescents and sclerosed glomeruli, type I collagen was co‐localized with type III collagen. The staining intensity of type I collagen in those areas was generally stronger than that in the interstitium. Mesangial staining for type I collagen was Present within the glomeruli, particularly with a marked mesangial matrix increase, but was less in amount and frequency compared with type III collagen staining.These findings indicate that the fibrosclerotic process in damaged glomeruli is accompanied by the appearance of interstitial collagens, and that participation of type I collagen is prominent in crescent organization and global glomerular sclerosis, but is less frequent in mesangial expansion, compared with type III collagen.