To evaluate the role of macrophages in glomerulonephritis, the relationship between the existence of macrophages and proteinuria in the long course of glomerulonephritis was studied in accelerated Masugi nephritis in the rat. Non-specific esterase staining was used as a marker of macrophages and the kinetics of glomerular macrophages was analysed by an image processor. Macrophages became detectable 48 hours after nephrotoxic serum injection and their accumulation reached a maximum level at 5 days. At 1 month, they had clearly decreased. At 3 months, however, many macrophages were found within the capsular drop-like lesions and crescents. Urinary protein excretion took roughly the same time course as the number of macrophages for 2 weeks. However, at 1 month, whereas macrophages had decreased, the proteinuria kept a persistently high level. At 3 months it showed a tendency to decrease. The correlation between the rate of appearance of macrophages and the urinary protein excretion assessed in 20 cases exhibited no significance. In addition, discrepancy in their time course was evident at 1 month. Although macrophages may happen to contribute to the tissue injury resulting in proteinuria in the early phase of nephrotoxic serum nephritis, factors other than macrophages should be considered in the subsequent duration of proteinuria in this model.