Background: Chronic long-term irradiation of mice with low dose-rate (20 mGy/day) gamma rays significantly shortened their life spans due to neoplasms and significant suppression of their ability to eliminate transplanted tumors. Materials and Methods: Mice exposed to chronic 20 mGy/day gamma rays for 400 days and housed in an enriched environment (EE) were compared with those housed in a standard environment. Results: We demonstrate that an EE significantly ameliorates the suppression of transplanted tumors in mice exposed to chronic low dose-rate gamma rays. Peripheral blood analysis revealed that the proportions of some immune cells were increased by EE housing. Additionally, the number and activity of natural killer (NK) cells in the peripheral blood increased with EE housing, indicating that NK cells may be involved in enhancing the ability to reject transplanted tumors. Conclusion: To our knowledge, this is the first study to report that the adverse effects, such as the suppression of tumor rejection activity of chronic low dose-rate irradiation exposure, may be alleviated by EE housing, which suggests the possibility that the adverse effects of irradiation may be mitigated by changes in lifestyle.