Myeloid dendritic cells (DC) and macrophages evolve from a common precursor. However, factors controlling monocyte differentiation toward DC or macrophages are poorly defined. We report that the surface density of the GM-CSF receptor (GM-CSFR) alpha subunit in human peripheral blood monocytes varies among donors. Although no correlation was found between the extent of GM-CSFR and monocyte differentiation into DC driven by GM-CSF and IL-4, GM-CSFR expression strongly influenced the generation of CD1a(+) dendritic-like cells in the absence of IL-4. CD1a(+) cells generated in the presence of GM-CSF express CD40, CD80, MHC class I and II, DC-SIGN, MR, CCR5, and partially retain CD14 expression. Interestingly, they spontaneously induce the expansion of CD4(+) and CD8(+) allogeneic T lymphocytes producing IFN-gamma, and migrate toward CCL4 and CCL19. Upon stimulation with TLR ligands, they acquire the phenotypic features of mature DC. In contrast, the allostimulatory capacity is not further increased upon LPS activation. However, by blocking LPS-induced IL-10, a higher T cell proliferative response and IL-12 production were observed. Interestingly, IL-23 secretion was not affected by endogenous IL-10. These results highlight the importance of GM-CSFR expression in monocytes for cytokine-induced DC generation and point to GM-CSF as a direct player in the generation of functionally distinct DC.