SummaryHuman dendritic cells (DC) can now be generated in vitro in large numbers by culturing CD34 + hematopoietic progenitors in presence of GM-CSF+TNFet for 12 d. The present study demonstrates that cord blood CD34 + HPC indeed differentiate along two independent DC pathways. At early time points (day 5-7) during the culture, two subsets of DC precursors identified by the exclusive expression of CDla and CD14 emerge independently. Both precursor subsets mature at day 12-14 into DC with typical morphology and phenotype (CDS0, CD83, CD86, CD58, high HLA class II). CDla + precursors give rise to cells characterized by the expression of Birbeck granules, the Lag antigen and E-cadherin, three markers specifically expressed on Langerhans cells in the epidermis. In contrast, the CD14 + progenitors mature into CDla + DC lacking Birbeck granules, E-cadherin, and Lag antigen but expressing CD2, CD9, CD68, and the coagulation factor XIlla described in dermal dendritic cells. The two mature DC were equally potent in stimulating allogeneic CD45RA + naive T cells. Interestingly, the CD14 + precursors, but not the CDla + precursors, represent bipotent cells that can be induced to differentiate, in response to M-CSF, into macrophage-like cells, lacking accessory function for T ceils.Altogether, these results demonstrate that different pathways of DC development exist: the Langerhans cells and the CD14+-derived DC related to dermal DC or circulating blood DC. The physiological relevance of these two pathways of DC development is discussed with regard to their potential in vivo counterparts.