The function of the mononuclear phagocyte system was assessed in vivo in 85 patients with primary and secondary glomerulonephritis, by measuring the clearance of IgG - sensitised 51Cr-labelled autologous erythrocytes. Eleven per cent of patients in clinical remission were found to have a delayed clearance, whereas impaired macrophage function was present in 62.5 per cent of the patients with major urinary abnormalities. Blockade of mononuclear phagocyte system, induced at least in part by unidentified factors, might have a role in development and perpetuation of glomerular injury.