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Publisher Summary This chapter reviews the role of Fc-receptor (FcR)-possessing cells on the destruction of human erythrocytes (E) sensitized with iso- and auto-antibodies of the IgG class. It focuses on the mechanism of monocyte-FcR-mediated lysis (MFL) and phagocytosis of E sensitized with IgG iso-antibodies anti-D (D-anti-D system). The chapter describes an experiment in which preparation of monocytes from the peripheral blood of healthy human volunteers, and Cr-labeling and sensitizing of E were done by standard methods. Monocytes and E were incubated at a ratio of 1:1 for 16–20 hours at 37°C. Erythrocytolysis was measured as percentage 51 Cr release into the supernatant. Subsequently, erythrophagocytosis was determined by counting the radioactivity remaining in the monocyte pellet after water lysis. Concerning the mechanism of MFL, it is assumed that lysosomal enzyme release (LER) is an important feature of this process. For this reason, experiments were performed with cytochalasin B (CB) that has the ability, among other things, to enhance LER specifically. It is logical to perform experiments with an inhibitor of LER, and hence, colchicine was used for this experiment. At a molarity of 10 -2 , colchicine completely blocked MFL and CB effect on MFL. At 10 -3 M colchicines, there was partial restoration of MFL and the CB effect. However, full restoration of MFL and the CB effect on MFL occurred only at 10 -4 M colchicine. Remarkably, phagocytosis still inhibited 75% at the concentration of colchicine. This study concludes that there exists heterogeneity in FcR avidity among the monocytes of an individual. Only monocytes with a high FcR avidity are active in MFL,and MFL is dependent upon LER but is independent from phagocytosis.