Dendritic cell (DC) development begins in the bone marrow but is not completed until after immature progenitors reach their sites of residence in lymphoid organs. The hematopoietic growth factors regulating these processes are poorly understood. Here we examined the effects of signaling by the receptor tyrosine kinase Flt3 on macrophage DC progenitors in the bone marrow and on peripheral DCs. We found that the macrophage DC progenitor compartment was responsive to superphysiological amounts of Flt3 ligand but was not dependent on Flt3 for its homeostatic maintenance in vivo. In contrast, Flt3 was essential to the regulation of homeostatic DC development in the spleen, where it was needed to maintain normal numbers of DCs by controlling their division in the periphery.