Dendritic cells (DCs) are rare but ubiquitous antigen-presenting cells situated in lymphoid and nonlymphoid organs throughout the body. The study of DCs located in the liver has been restricted by their relative scarcity and the difficulty of their isolation. Because granulocyte-macrophage colony-stimulating factor (GM-CSF) is a critical growth factor for DCs in vitro, we postulated that it would expand hepatic DCs in vivo. We found that adenoviral-mediated GM-CSF overexpression in normal mice increased the number of liver DCs 400-fold to more than 100 million cells. GM-CSF-recruited DCs were CD11c(+)DEC205(-) and had high expression of major histocompatibility complex (MHC) class II, CD54, and CD80 but low CD40 and CD86 staining. Further maturation occurred after overnight culture. In addition to CD11c(+)DEC205(-) DCs, a population of CD11c(-)DEC205(low/-) cells resembling DC progenitors described previously in normal mice was expanded as serum GM-CSF levels increased. GM-CSF-recruited CD11c(+)DEC205(-) DCs and CD11c(-)DEC205(low/-) cells had different functional capabilities. CD11c(+)DEC205(-) DCs captured far more protein antigen in vivo, produced higher amounts of interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha, and induced greater allogeneic and antigen-specific T-cell stimulation. A proportion of CD11c(-)DEC205(low/-) cells differentiated into CD11c(+) cells and gained T-cell stimulatory ability when cultured in the presence of GM-CSF. In conclusion, our findings show that GM-CSF can profoundly influence recruitment and development of DCs in murine liver.