Bone marrow transplants are an important therapeutic tool for treating certain types of cancer as well as genetic diseases affecting the hematopoietic system. Until the transferred stem cells differentiate and reconstitute the immune system, recipients are at increased risk from opportunistic infections. We report the rapid generation of a functional natural killer (NK) compartment in lethally irradiated mice that received bone marrow cells from a syngeneic donor by treatment with IL-2/anti-IL-2 antibody complexes. We demonstrate that IL-2 complexes specifically expand the donor but not the host NK population and discuss the implications of this finding in the context of graft-versus-host disease and tumor relapse. Finally, we show that NK cells rapidly generated by IL-2 complexes kill MHC class I-deficient cells effectively in vivo. These data underline the unique therapeutic potential of IL-2 complexes.