Type IV collagen is a major component of the renal glomerular extracellular matrix. A recently characterized monoclonal antibody, JK132, which was originally produced by immunization with human placental type IV collagen, recognizes a new epitope which is different from alpha 1-alpha 6 chains of type IV collagen. Using immunofluorescence and immunogold electron microscopy, the distribution of the epitope of JK132 has been compared with the distribution of alpha 1, alpha 2, alpha 3 and alpha 4 chains of type IV collagen in normal human kidney and in the renal tissues of patients with various types of glomerulonephritis. In normal human kidney, JK132 reacted with mesangial matrix, Bowman's capsular basement membrane (BCBM), tubular basement membrane, and vessel walls, but did not react with glomerular basement membrane (GBM). This distribution is different from the distribution of alpha 1-alpha 4(IV) chains. In IgA nephropathy and membranoproliferative glomerulonephritis, the staining intensity for JK132 was increased in expanded mesangial matrix. In glomeruli with severe mesangial proliferation, the epitope of JK132 extended to the endothelial side of the GBM. In membranous nephropathy, staining for JK132 was virtually unchanged from normal. This study suggests that the epitope of JK132 increases in amount during the process of mesangial proliferation and could serve as a marker for mesangial matrix expansion in glomerulonephritis.