MP84 is a novel antigen expressed in the primary glomerulonephritis kidney biopsy section and stimulated mesangial cells and matrix but not in the normal kidney and other tissue sections or any other cell lines tested except Bewo choriocarcinoma cell line. This is the first report of MP84 expression in pyelonephritis and hydronephrosis kidney sections. The distribution of the antigen is different from the primary glomerulonephritis kidney sections studied previously such as membranous, minimal change, focal and segmental, and IgA nephropathy. This may have an important correlation with the pathological concept underlying the disease.