Recent studies in experimental glomerulonephritis (GN) suggest that the monocyte is involved in the clearance of detritus and deposits from glomeruli. Whether this concept applies to human GN is unclear because of the paucity of studies in this area. Histochemical evaluation of more than 300 human renal biopsies disclosed eight cases of diffuse proliferative GN which demonstrated relatively large numbers of intraglomerular nonspecific esterase-positive cells. The diagnoses included GN associated with mixed essential cryoglobulinemia (three cases), diffuse lupus GN (three cases), and idiopathic diffuse proliferative GN (two cases). Electron microscopy disclosed extensive deposition of electron-dense material along glomerular basement membranes. Monocytes were present within capillary lumina, were usually associated with visible deposits, and often insinuated themselves between endothelium and the deposits so as to abut the deposits directly. The cells showed evidence of maturation (activation). The contents of the secondary lysosomes were generally similar in structure and density to neighboring deposits, especially in one case in which the deposits had an organized structure. The findings suggest that these monocytes are involved in the clearance of deposits from the glomerulus.