Total body irradiation (TBI), given in low to moderate doses as a single modality, can enhance leukocyte populations and immune modifiers, resulting in slowed tumor progression. The aim of this study was to evaluate natural killer (NK) cell involvement in mediating the antitumor effect of TBI by depleting NK populations and monitoring tumor progression and immune status following exposure. C57BL/6 mice (n=54) were injected with anti-NK1.1, anti-asialo GM1, or rabbit serum prior to irradiation/tumor implantation. Selected animal groups were irradiated with a 3 Gy dose of gamma-rays and Lewis lung carcinoma (LLC) cells were subcutaneously implanted 2 h later. Tumor volumes, leukocyte populations, and cytokine levels in blood and spleen were measured up to 10 days post-irradiation/tumor implantation. Depletion of asialo GM1+ cells, but not NK1.1+ cells, led to significant acceleration of tumor growth (P<0.05). Challenge with exogenous antigens (rabbit antibodies or serum) when accompanied by administration of TBI resulted in: a) radioresistance of splenic lymphocytes, b) increased granulocyte and monocyte numbers, and c) enhanced production of IgG, IL-10, and IL-18 within plasma and tumor supernatants. Delivery of TBI to NK1.1+ depleted mice, did not show similar enhancement of leukocytes and/or their modulators. These data indicate that TBI, in conjunction with immune challenge, activates leukocyte parameters and redirects the immune system toward a T helper 2 (Th2) cell response. Additionally, NK cells are involved in mediating the antitumor effect of TBI, while challenge with exogenous protein attenuates the slowing of malignant growth that accompanies delivery of radiation. These findings also support the premise that radiation exposure can activate NK and some T cytotoxic lymphocytes, thereby leading to tumor suppression.