Ultrastructural cytochemistry for nonspecific esterase (NSE) was performed on normal and nephritic rabbit kidneys. In normal glomeruli distinct reaction deposits appearing as electron-dense granules were present in visceral epithelial cells (podocytes) and epithelial cells of Bowman's capsule, but only a few small or minute deposits were seen in some endothelial and mesangial cells. In acute proliferative glomerulonephritis (Masugi nephritis), many reaction deposits occurred in mononuclear cells accumulating in glomerular tufts which presented the characteristic features of monocytic cells. Macrophages which had migrated into subendothelial space as well as epithelioid cells and multinucleated giant cells, all of which are known to be derived from monocytes, also exhibited the reaction product. The NSE granules in mesangial and endothelial cells were much smaller and fewer in number than those in monocytic cells. The present method may contribute to the more precise differentiation of monocytic cells from mesangial and endothelial cells in proliferative glomerulonephritis.