Glomerular localization of type III collagen in human kidney disease. Kidney specimens taken from normal humans and patients with various renal diseases were examined by immunofluorescent and immunoelectron microscopy using a monoclonal antibody to the α, chain of type III collagen. Indirect immunofluorescent staining revealed that intraglomerular localization of type III collagen antigen in 41 of 66 patients, while it was absent from the glomeruli of normal human kidneys. Type III collagen was found within the mesangium of 22 patients with various types of renal diseases, and was distributed in a focal and segmental manner in most of the cases. Mesangial localization of the collagen correlated with the increase in the mesangial matrix. Type III collagen was also present in the vascular pole, crescents (particularly in the organizing phase) and sclerosed glomeruli. Immunoelectron microscopy using pre-embedding and post-embedding techniques confirmed the above observations. These findings indicate that type III collagen participates in mesangial expansion, crescent organization, and glomerulosclerosis.