Interactions mediated by TCRs expressed on different T cell subsets may play a role in immunoregulation. To investigate this idea, we studied the regulation of superantigen-induced TCR Vβ-restricted responses. We asked whether the in vivo regulation of CD4⁺ Vβ8⁺ T cells following SEB injection is controlled by CD8⁺ T cells. We found that in mice deficient in CD8⁺ T cells, the down-regulation of CD4⁺ Vβ8⁺ T cells below baseline is not observed. Moreover, following SEB administration, CD8⁺ T cells emerge that preferentially kill subpopulations of activated CD4⁺ Vβ8⁺ but not CD4⁺ Vβ8^- T cells in vitro. This TCR Vβ-specific cytotoxicity is dependent on β2-microglobulin and is inhibited by antisera specific for Qa-1 but not by antibody to MHC class la. These data suggest the idea that the specificity of immune regulation may involve CD8⁺ T cell recognition of TCR Vβ determinants and Qa-1 molecules expressed on CD4⁺ T cells.