The cytokine FLT3 ligand (FL) enhances dendritic cell (DC) generation and has therefore been proposed as a means to boost antitumor immunity. Vascular endothelial growth factor (VEGF) is produced by a large percentage of tumors and is required for development of tumor neovasculature. We previously showed that VEGF decreases DC production and function in vivo. In this study, we tested the hypothesis that VEGF regulates FL effects on DC generation. In seven experiments, four groups of mice were treated with PBS, VEGF alone (100 ng/h), FL alone (10 microgram/day), or with the combination of FL and VEGF. VEGF and PBS were administered continuously for 14 days via s.c. pumps. FL was given s.c. daily for 9 days, beginning on day 4. Tissues were collected and the number, phenotype, and function of lymph node, splenic, and thymic DCs were analyzed on day 14. As expected, treatment with FL resulted in a marked increase in the number of lymph node and spleen DCs and a smaller increase in thymic DC. Pretreatment of mice with VEGF inhibited these FL effects in lymph nodes and thymus by about 50%, whereas spleen DC numbers were undiminished by VEGF. VEGF treatment in vivo also inhibited the ability of FL to increase the number of hemopoietic precursor cells and the level of maturity exhibited by DC derived from these hemopoietic precursor cells in vitro. VEGF inhibited FL-inducible activation of transcription factor NF-kappaB. These data suggest that VEGF interferes with the ability of FL to promote dendritic cell differentiation from bone marrow progenitor cells in mice and therefore may decrease the therapeutic efficacy of FL in settings where increased numbers of DCs might provide clinical benefits.