Abstract Immunotherapy aiming at the combined activation of tumor associated antigen (TAA) specific cytotoxic T lymphocytes (CTL) and Natural Killer (NK) cells may be crucial to eradicate both MHC-I positive and negative tumors. Vaccination with mature dendritic cells (DC) transfected with mRNA encoding for TAA and the pro-inflammatory cytokines interleukin (IL)-12 and IL-18 may increase NK cell and TAA specific CTL activity. We demonstrate here that IL-12 over-expressing human DC induces increased NK cell activation and effector function and confirm the increase in TAA specific CTL by TAA/IL-12 double transfected DC. The effects of IL-18 transfection were limited to phenotypic activation and down-regulation of tissue homing receptors and did not add to the effect of IL-12 on NK cell effector function. In conclusion, co-transfection of TAA and IL-12 mRNA into mature DCs offers a vaccine for the induction of an anti-tumor immune response mediated by CTL and NK effector cells.