The local cytokine environment and the presence of stimulatory signals determine whether circulating monocytes will finally acquire characteristics of dendritic cells (DCs) or macrophages. Because FcepsilonRI expressed on professional APCs, e.g., monocytes and DCs, has been suggested to play a key role in the pathophysiology of atopic diseases, we evaluated the effect of receptor ligation on the generation of monocyte-derived DCs (MoDCs). Aggregation of FcepsilonRI at the initiation of the IL-4-GM-CSF-driven differentiation resulted in the emergence of macrophage-like cells with a strong expression of the mannose receptor and a low level of CD1a and the DC-specific markers CD83 and the actin-bundling protein (p55). These cells sustained the ability to take up FITC-labeled Escherichia coli by phagocytosis and were significantly less efficient in stimulating purified allogeneic T cells. In addition, receptor ligation of FcepsilonRI at the beginning of the culture prevented the generation of MoDCs, mainly due to a dramatic increase in the IL-10 production. These results suggest that FcepsilonRI aggregation prevents the generation of CD1a(+) MoDCs and imply a novel pivotal function of this receptor in modulating the differentiation of monocytes.