Mismatched hematopoietic cell transplants for treating leukemia are complicated by graft versus host disease (GvHD). Here, we show that adoptively transferred IL-12/15/18-preactivated NK cells suppress GvHD in a mouse model of fully mismatched hematopoietic cell transplantation. These IL-12/15/18-preactivated NK cells maintained Eomesodermin (Eomes) and T-bet expression upon transfer and, while there was no evidence of direct killing of donor T cells or host DCs by the IL-12/15/18-preactivated NK cells, proliferation of donor T cells was inhibited. Strikingly, the graft versus leukemia effect mediated by donor T cells was retained, resulting in improved overall survival of mice that received lymphoma cells, donor allogeneic T cells, and IL-12/15/18-preactivated NK cells. These results suggest that IL-12/15/18-preactivated NK cells may be useful in improving immunotherapy of mismatched hematopoietic cell transplantation. Compared with previously proposed protocols, our findings suggest that in vitro NK-cell preactivation with this cytokine cocktail offers the significant advantage that cytokines do not need to be administered systemically to sustain NK-cell activity, thus avoiding toxicity.