CD4 CD25 regulatory T cells (Treg cells) are an important subset of T cells for keeping proper immune responses and tolerance. However, the effects of γ radiation on CD4 CD25^high Foxp3 Treg cells have not been examined previously. In the present study, we compared the sensitivity of mouse CD4 CD25^high Foxp3 Treg cells and CD4 CD25⁻ T cells to γ radiation in vitro and in vivo. After C57BL/6 mice received a whole-body dose of 5 Gy γ rays, the numbers of lymphocyte subsets in blood, lymph nodes, spleens and thymuses clearly decreased. However, γ radiation significantly enhanced the ratios of CD4 CD25^high Treg cells and CD4 CD25^high Foxp3 Treg cells to CD4 T cells in the blood, lymph nodes, spleens and thymuses of mice. More dead cells were observed in CD4 CD25⁻ T cells than in CD4 CD25^high Treg cells or CD4 CD25^high Foxp3 Treg cells when the cells were irradiated in vitro, indicating that CD4 CD25^high Foxp3 Treg cells are more resistant to γ radiation than other T cells. Moreover, a higher expression of Bcl-2 in CD4 CD25^high Treg cells was detected compared with that in CD4 CD25⁻ T cells. CD4 CD25 Treg cells from irradiated mice were functional, though their immunosuppressive ability was somewhat impaired compared to those from nonirradiated mice as determined by an in vitro assay. These results indicate that mouse CD4 CD25 Treg cells and CD4 CD25⁻ T effector cells have different sensitivities to γ radiation in mice.