The aim of this study was explore the effect of natural killer (NK) cells on engraftment and reconstitution of hematopoiesis and immunity in mice undergoing allogeneic bone marrow transplantation (allo-BMT). Lethally and nonlethally irradiated BALB/c (H-2(d)) mice were transplanted with C57BL/6 (H-2(b)) bone marrow plus donor peripheral T cells and/or NK cells. Recipient CD34(+), H-2K(b+), CD3(+) and CD19(+) cells were detected by flow cytometry; peripheral blood leukocytes were counted by auto-cytometry; survival rates, engraftment, hematopoietic and immune recovery of recipients in different transplant groups were then observed. The results indicated that as compared with lethally irradiated and allo-BMT group without infusion of NK cells, the survival rate in lethally irradiated and allo-BMT group with infusion of NK cells significantly enhanced (survival rates at 60 days were 70% and 0.0% respectively); leukocyte count, expression level of CD19(+) cell and CD34(+) cell count recovered rapidly; expression level of H-2K(b+) cells obviously increased [(86.68 +/- 4.45)% vs (4.68 +/- 0.32)%]; expression level of CD3(+) cell at day 28 after transplantation obviously decreased [(33.69 +/- 3.36)% vs (50.40 +/- 5.06)%, p < 0.01], at day 60 there was not significant difference between the 2 groups (p > 0.05). In nonlethally irradiated and allo-BMT group without NK cell infusion, the expression level of H-2K(b+) at day 30 after transplantation significantly decreased, and reduced to level before transplantation at day 60; while expression of H-2K(b+) yet could be detected with > 80% at day 60 after transplantation in group infused with high and low concentration of NK cells. It is concluded that in allo-BMT mice, alloreactive NK cell inhibits graft rejection, enhances engraftment, promotes the reconstitution of hematopoiesis and immunity, and increases survival rates.