To clarify the relationship between macrophages and development of glomerulosclerosis, the authors developed a new experimental nephritis model with macrophages persisting in Thy-1 nephritis. Methyl-cellulose was administered intraperitoneally in addition to the intravenous injection of the anti-Thy-1 antibody to Wistar rats. Foamy macrophages influxed into the lytic mesangium and stayed to form nodular aggregates. Mesangial cells proliferated with the formation of extracellular matrices around these nodular aggregates of macrophages. Immunohistochemical analyses revealed that alpha-smooth muscle actin (alpha-SMA) was expressed in the proliferative area around these nodules of foamy macrophages from day 7. Type I collagen and type IV collagen were also expressed around the foamy macrophages in correspondence with alpha-SMA expression from day 7. The electron microscopic study revealed that collagen fibrils were formed around the transformed mesangial cells. The expression of platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31), a marker of glomerular vasculature endothelial cells, was not found in the area occupied by the foamy macrophages, suggesting the impairment of glomerular reconstruction. Macrophages may participate in the progression of glomerulosclerosis in Thy-1 nephritis by enhancing the production of the extracellular matrix through transformed mesangial cells and preventing reconstruction of the capillary network.