Fms-like tyrosine kinase 3 ligand ([Flt3 ligand], FL) stimulates proliferation and development of a wide range of hematopoietic cells including hematopoietic stem cells and myeloid and lymphoid progenitor cells. FL also has been shown to have anti-tumor effects in a variety of in vivo tumor models. In this study, the effect of FL against tumor growth was investigated in the COLO-205 human colon tumor xenograft model. FL was delivered in vivo by the “hydrodynamics-based gene delivery of naked DNA” method. In this experimental setting, FL and/or the therapeutic antibody anti-carcinoembryonic antigen (CEA) monoclonal antibody was administered. FL alone or anti-CEA antibody alone induced significant growth inhibition; furthermore, FL plus antibody treatment produced synergistic anti-tumor effects. This study is the first demonstration of a synergistic anti-tumor effect between FL and antibody therapeutics.