The effects of the reduction of neutrophils in glomeruli on the improvement of proteinuria and glomerular injuries were determined in the first (heterologous) phase of Masugi (nephrotoxic) nephritis. Male (6-week-old) WKA/Hkm rats were initially injected with 2.0 ml of a newly developed monoclonal antineutrophil antibody and then injected with 1.0 ml of nephrotoxins. This monoclonal anti-neutrophil antibody was found to have selectively reduced the number of neutrophils in the glomerular capillary lumen in cases of Masugi nephritis by light microscopy. Malondialdehyde (MDA) levels or superoxide dismutase (SOD) activities in renal tissues of such rats were also examined. However, there were no significant differences in the levels of proteinuria and the number of glomerular cells containing resident cells and infiltrated mononuclear cells in the first phase of Masugi nephritis with or without pretreatment with antineutrophil antibody. No significant differences were observed in the levels of MDA or SOD activities in renal tissues of Masugi nephritis with or without pretreatment with such an antibody either. It appeared that infiltration of neutrophils in the glomeruli might not be related to proteinuria and glomerular injuries in the first phase of Masugi nephritis. It was postulated that the massive proteinuria in the first phase of Masugi nephritis might be correlated with the activities of reactive oxygen species induced by the glomerular cells, i.e. glomerular resident cells and infiltrated mononuclear cells.