The Fc-receptor function of the mononuclear phagocyte system was examined in 30 patients affected by multisystem diseases with glomerular involvement by measuring the immune clearance of IgG-sensitized autologous red blood cells in vivo and the immune phagocytosis of monocytes in vitro. Patients studied in the phase of clinically active renal disease showed a significantly reduced Fc-receptor function by both in vivo (p less than 0.001) and in vitro (p = 0.003) assays, as compared to those studied during an inactive phase. Though the nature of the defect remains uncertain, it appears to be related to the active phase of the renal disease as also confirmed by the analysis of individual cases studied longitudinally.