Natural killer (NK) cells are composed of subsets characterized by the expression of inhibitory or activating receptors, or both, specific for different major histocompatibility complex (MHC) class I determinants. We have previously shown that inhibitory receptor blockade of syngeneic NK cells was an effective means of ex vivo purging of leukemia-contaminated bone marrow and that the transplantation of mice with the purged bone marrow cells (BMCs) resulted in long-term, relapse-free survival. We have extended the investigation to assess the antitumor effects mediated by NK cells H2-allogeneic to tumor cells. We demonstrate that various tumor cell lines are more susceptible to lysis by H2-allogeneic NK cells than by syngeneic NK cells in vitro even though comparable percentages of Ly49 NK cells were present. Using allogeneic NK cells to purge leukemia-contaminating BMCs before transplantation resulted in a higher proportion of mice with long-term survival than using syngeneic NK cells. Allogeneic NK cells did not suppress hematopoietic reconstitution as measured by granulocyte/monocyte-colony-forming unit (CFU-GM), complete blood count (CBC), and donor chimerism at various days after transplantation. Inhibitory receptor blockade of allogeneic NK cells also significantly increased these antitumor effects at lower NK/tumor ratios compared with those of syngeneic NK cells. These results demonstrate that H2-allogeneic NK cells mediate more potent antitumor effects than syngeneic NK cells without adverse hematologic effects and thus may be useful in cancer therapy.