Studies of human NK cells and their role in tumor suppression have largely been restricted to in vitro experiments which lack the complexity of whole organisms, or mouse models which differ significantly from humans. In this study we showed that, in contrast to C57BL/6 Rag2(-/-)/gamma(c) (-/-) and NOD/Scid mice, newborn BALB/c Rag2(-/-)/gamma(c) (-/-) mice can support the development of human NK cells and CD56+ T cells after intrahepatic injection with hematopoietic stem cells. The human CD56(+) cells in BALB/c Rag2(-/-)/gamma(c) (-/-) mice were able to produce IFN-gamma in response to human IL-15 and polyI:C. NK cells from reconstituted Rag2(-/-)/gamma(c) (-/-) mice were also able to kill and inhibit the growth of K562 cells in vitro and were able to produce IFN-gamma in response to stimulation with K562 cells. In vivo, reconstituted Rag2(-/-)/gamma(c) (-/-) mice had higher survival rates after K562 challenge compared to non-reconstituted Rag2(-/-)/gamma(c) (-/-) mice and were able to control tumor burden in various organs. Reconstituted Rag2(-/-)/gamma(c) (-/-) mice represent a model in which functional human NK and CD56+ T cells can develop from stem cells and can thus be used to study human disease in a more clinically relevant environment.