Flt3 ligand, a recently described growth factor affecting early hematopoietic progenitor cells, can also support the expansion of dendritic cells secreting IL-12. Its potential use in a clinical setting has been suggested. Here, we studied the effect of in situ delivery of Flt3 ligand plasmid (FL) on the antibody response induced by DNA vaccine encoding wild-type hepatitis B virus core antigen (HBc/w). Intramuscular injection of FL increased the expression of DEC205 and the size of splenocytes, and immunization with HBc/w or HBc/w-transfected EL-4 cells induced strong anti-HBc antibody responses in mice. However, intramuscular injection of FL with HBc/w significantly suppressed HBc/w-induced antibody response in a dose-dependent manner. Suppression of immune response by FL injection was the most prominent when FL and HBc/w were co-injected at the same time and the same site. These results suggest that FL may inhibit humoral response induced by DNA-type vaccination, and DC locally expanded by FL may not have proper functions for induction of humoral response.