Antineutrophil cytoplasmic autoantibodies (ANCAs) are found in the sera of patients with systemic vasculitis and crescentic glomerulonephritis. We developed monoclonal antibodies against myeloperoxidase (MPO), which is a major perinuclear-ANCA antigen, and examined the presence of MPO in renal tissue from patients with rapidly progressive glomerulonephritis (RPGN) using these monoclonal antibodies. Myeloperoxidase was found in the glomeruli in four of five cases of perinuclear ANCA-positive RPGN, and the distribution pattern was different from that of immunoglobulin G. In perinuclear ANCA-negative crescentic glomerulonephritis, MPO also was detected in two of three cases. Myeloperoxidase was not detected in the capillary walls or mesangium in other nephropathies, such as minimal change disease, membranous nephropathy, and mesangial proliferative glomerulonephritis, or in normal controls. Myeloperoxidase activity was elevated and inversely correlated with the titer of MPO-ANCAs in the sera of perinuclear ANCA-associated RPGN. These data suggest that MPO plays an important role in the glomerular injury of RPGN.