Dendritic cells (DCs) are rare antigen-presenting cells that play a central role in stimulating immune responses. The combination of recombinant granulocyte macrophage–colony-stimulating factor (rGM-CSF) and recombinant interleukin-4 (rIL-4) provides an important stimulus for generating DCs from murine bone marrow precursors in vitro. Using miniature osmotic pumps, we now demonstrate that continuous infusion of these cytokines for 7 days had a similar effect in vivo, increasing the number and function of splenic DCs. Administration of rGM-CSF/rIL-4 (10 μg/d each) increased the concentration of CD11⁺ DCs by 2.7-fold and the absolute number of splenic DCs by an average of 5.7-fold. DC number also increased in peripheral blood and lymph nodes. The resultant DCs exhibited a different phenotype and function than those in control mice or mice treated with rGM-CSF alone. rGM-CSF/IL-4 increased both the myeloid (CD11c⁺/CD11b⁺) and the lymphoid (CD11c⁺/CD8α⁺) subpopulations, whereas rGM-CSF increased only myeloid DCs. DCs were highly concentrated in the T-cell areas of white pulp after rGM-CSF/IL-4 administration, whereas they were diffusely distributed throughout white pulp, marginal zones, and red pulp in mice treated with rGM-CSF alone. rGM-CSF/rIL-4 also significantly increased the expression of major histocompatibility complex (MHC) class I and MHC class II on CD11c⁺ cells and increased their capacity to take up antigens by macropinocytosis and receptor-mediated endocytosis. Splenic DCs generated in response to rGM-CSF/rIL-4 were functionally immature in terms of allostimulatory activity, but this activity increased after short-term in vitro culture. Systemic treatment with rGM-CSF/rIL-4 enhanced the response to an adenoviral-based vaccine and led to antigen-specific retardation in the growth of established tumor. We conclude that systemic therapy with the combination of rGM-CSF/rIL-4 provides a new approach for generating DCs in vivo.