Suppression by regulatory T cells is now acknowledged to play a key role in the down-regulation of T cell responses to foreign and self Ags. In addition to the naturally occurring CD4+ CD25+ population, several subtypes of induced regulatory cells have been reported, but their mechanisms of action remain unclear. Conversely, cytotoxic CD4+ cells that lyse cells presenting their cognate peptide have been described, but their potential role in immunoregulation remains to be delineated. A CD4+ T cell line derived from BALB/c mice immunized with peptide 21–35, containing a major T cell epitope of a common allergen, Dermatophagoides pteronyssinus group 2 allergen, was found to lyse the Ag-presenting WEHI cell line via Fas-Fas ligand and only in the presence of the cognate peptide. Cytolytic activity was likewise shown for other T cell lines and occurred even after a single cycle of in vitro stimulation. Moreover, T cells that efficiently lysed WEHI cells were unresponsive to stimulation with their cognate Ag and were dependent on IL-2 for growth and survival, which was reflected in a constitutive expression of CD25 independently of activation status. Proliferating B cells were also killed by the CTLs. By lysing Ag-presenting B cells in an epitope-specific manner, the nonproliferating CTLs were shown to down-regulate the proliferation of bystander T cells. These data demonstrate that cytotoxic CD4+ CD25+ T cells that lack proliferation capacities have the potential to down-regulate an immune response by killing Ag-presenting B cells. This could represent an important and specific down-regulatory mechanism of secondary immune responses in vivo.