To investigate a possible role of anti-neutrophil cytoplasmic antibodies directed against myeloperoxidase (MPO-ANCA) in glomerulonephritis, we prepared anti-rat MPO antiserum by immunization of rat MPO into a rabbit. Then we administered anti-rat MPO antiserum (group 1) or normal rabbit serum (NRS) (group 2) into rats before injection of nephrotoxic serum (NTS), which induced nephrotoxic serum nephritis (NTN). Other groups of rats received either anti-rat MPO anti-serum (group 3) or NRS (group 4) before injection of NRS but not NTS. Rats in group 1 and group 2 were sacrificed at either 3 hours, 15 hours, or 14 days after NTS injection. Rats in group 3 and group 4 were sacrificed at 15 hours after the last NRS injection. By light microscopy, in rats with NTN sacrificed at 3 hours, counts of polymorphonuclear leukocytes (PMN) per glomerulus were 21.6 +/- 3.5 in group 1 and 8.4 +/- 1.7 in group 2 (P < 0.01). At 15 hours, massive glomerular fibrin deposits were observed in group 1 rats (fibrin score, 131 +/- 8), but not in group 2 rats (fibrin score, 27 +/- 21; P < 0.01). By direct immunofluorescence microscopy, rat MPO was found along glomerular capillary walls more intensely in group 1 rats than in group 2 rats. No pathological alterations were found in group 3 and group 4 rats. Further, renal elution studies revealed that eluted rabbit IgG contained anti-rat MPO antibodies in group 1 rats but not in group 3 rats. These results suggest that the anti-MPO antibodies are directly involved in the more severe glomerular lesions in group 1 rats via interactions with MPO itself or activation of PMN, which release various kinds of mediators including MPO.