The mononuclear phagocyte system, formerly called the reticuloendothelial system, is an important element in basic immunology, cell biology, and clinical disease. Secretory products participate in inflammation and immunoregulation. Endocytosis mediated by specific receptors for immunoglobulin and complement or by other opsonins is important in removal of damaged self or foreign particles. The ability to assess receptor-specific endocytosis has led to the recognition of Fc-receptor dysfunction in certain autoimmune diseases. This defect in membrane receptor function, whether inherited or acquired, may be important in the pathogenesis of these diseases. © 1983.