The aim of this study was to examine the contribution of local proliferation in the development of macrophage accumulation and macrophage-mediated injury in rat anti-GBM glomerulonephritis. Using double immunohistochemistry staining of monocyte/macrophages plus the proliferating cell nuclear antigen (PCNA) or bromodeoxyuridine (BrdU) incorporation, we found that the initial accumulation of ED1+ macrophages in the kidney on day 1 of disease was due to an influx of circulating monocytes. However, large numbers of proliferating macrophages (ED1+PCNA+ cells), including mitotic macrophages, were present within the glomerulus and interstitium during disease progression (days 7 to 21), accounting for up to 62% of the total macrophage population and giving an excellent correlation with total macrophage accumulation (glomerulus, r = 0.92; interstitium, r = 0.94; both P < 0.001). These proliferating cells had a monocyte phenotype (ED1+ED2-ED3-), but this marked proliferative activity was restricted to the diseased kidney since no PCNA expression or BrdU incorporation was evident within circulating blood monocytes. Proliferating macrophages were almost exclusively localized in areas of severe tissue damage and they correlated significantly with glomerular and tubulointerstitial lesions (P < 0.001), proteinuria (P < 0.001) and creatinine clearance (P < 0.01). In marked contrast, glomerular PCNA- macrophages failed to correlate with these parameters. In conclusion, this study has demonstrated that local macrophage proliferation is the major mechanism of macrophage accumulation during the progression of rat anti-GBM glomerulonephritis. Furthermore, it suggests that proliferating macrophages are potent local effector cells in the mediation of progressive renal injury in this disease.