The Fas/FasL (CD95/CD178) system is required for immune regulation; however, it is unclear in which cells, when, and where Fas/FasL molecules act in the immune system. We found that CD8⁺CD122⁺ cells, which are mostly composed of memory T cells in comparison with naïve cells in the CD8⁺CD122⁻ population, were previously shown to include cells with regulatory activity and could be separated into CD49d^low cells and CD49d^high cells. We established in vitro and in vivo experimental systems to evaluate the regulatory activity of CD122⁺ cells. Regulatory activity was observed in CD8⁺CD122⁺CD49d^low but not in CD8⁺CD122⁺CD49d^high cells, indicating that the regulatory cells in the CD8⁺CD122⁺ population could be narrowed down to CD49d^low cells. CD8⁺CD122⁻ cells taken from lymphoproliferation (lpr) mice were resistant to regulation by normal CD122⁺ Tregs. CD122⁺ Tregs taken from generalized lymphoproliferative disease (gld) mice did not regulate wild-type CD8⁺CD122⁻ cells, indicating that the regulation by CD122⁺ Tregs is Fas/FasL-dependent. CD122⁺ Tregs taken from IL-10–deficient mice could regulate CD8⁺CD122⁻ cells as equally as wild-type CD122⁺ Tregs both in vitro and in vivo. MHC class I-missing T cells were not regulated by CD122⁺ Tregs in vitro. CD122⁺ Tregs also regulated CD4⁺ cells in a Fas/FasL-dependent manner in vitro. These results suggest an essential role of Fas/FasL as a terminal effector of the CD122⁺ Tregs that kill activated T cells to maintain immune homeostasis.