Abstract Compelling evidence indicate that regulatory T (Treg) cells play an important role in the maintenance of immune tolerance to self and foreign antigens (Ag). Various subsets of T lymphocytes have been isolated in mice and humans that have the ability to down-regulate the proliferation of autoimmune effector cells. Recently, a novel subset of Ag-specific T-cell receptor (TCR)αβ+ CD4−CD8− (double negative, DN) Treg cells has been found to be able to prevent the rejection of skin and heart allografts by specifically inhibiting the function of anti-graft-specific CD8+ T cells. Here we demonstrate that peripheral DN Treg cells are present in humans, where they constitute about 1% of total CD3+ T cells, and consist of both naïve and Ag-experienced cells. Furthermore, analysis of T-cell receptor excision circles (TRECs) indicate that DN T cells are not recent thymic emigrants, but rather an expanded T-cell subset. MHC multimer staining revelaed a distinct population of DN T cells recognizing common MHC class I-restricted CMV and EBV antigens. DN T cells exhibited a strong proliferative response upon stimulation with allogeneic antigen presenting cells (APC) and secreted high amounts of IFN-γ but no IL-2, with some IL-5, and marginal levels of IL-4 and IL-10. Similar to murine DN Treg cells, human DN Treg cells are able to acquire peptide-HLA-A2 complexes from APCs by cell contact-dependent mechanisms. Furthermore, such acquired peptide-HLA complexes appear to be functionally active, in that CD8+ T cells specific for the HLA-A2-restricted self peptide, Melan-A, became sensitive to apoptosis by neighboring DN T cells after acquisition of Melan-A-HLA-A2 complexes and revealed a reduced proliferative response. These results demonstrate for the first time that a sizeable population of peripheral DN Treg cells exists in humans that are able to suppress Ag-specific T cells. DN Treg cells may serve to limit clonal expansion of allo-Ag-specific T cells after transplantation.