Dendritic cells (DC) represent key regulators of the immune system, yet their development from hemopoietic precursors is poorly defined. In this study, we describe an in vitro system for amplification of a Flt3+ CD11b+ progenitor from mouse bone marrow with specific cytokines. Such progenitor cells develop into both CD11b+ and CD11b− DC, and CD8α+ and CD8α− DC in vivo. Furthermore, with GM-CSF, these progenitors synchronously differentiated into fully functional DC in vitro. This two-step culture system yields homogeneous populations of Flt3+ CD11b+ progenitor cells in high numbers and allows monitoring the consecutive steps of DC development in vitro under well-defined conditions. We used phenotypic and functional markers and transcriptional profiling by DNA microarrays to study the Flt3+ CD11b+ progenitor and differentiated DC. We report here on an extensive analysis of the surface Ag expression of Flt3+ CD11b+ progenitor cells and relate that to surface Ag expression of hemopoietic stem cells. Flt3+ CD11b+ progenitors studied exhibit a broad overlap of surface Ags with stem cells and express several stem cell Ags such as Flt3, IL-6R, c-kit/SCF receptor, and CD93/AA4. 1, CD133/AC133, and CD49f/integrin α 6. Thus, Flt3+ CD11b+ progenitors express several stem cell surface Ags and develop into both CD11b+ and CD11b− DC, and CD8α+ and CD8α− DC in vivo, and thus into both of the main conventional DC subtypes.