A subpopulation of peripheral human CD4 CD25 T cells that expresses CD45RO, histocompatibility leukocyte antigen DR, and intracellular cytotoxic T lymphocyte–associated antigen (CTLA) 4 does not expand after stimulation and markedly suppresses the expansion of conventional T cells in a contact-dependent manner. After activation, CD4 CD25 T cells express CTLA-4 on the surface detectable for several weeks. These cells show a G1/G0 cell cycle arrest and no production of interleukin (IL)-2, IL-4, or interferon (IFN)-on either protein or mRNA levels. The anergic state of CD4 CD25 T cells is not reversible by the addition of anti-CD28, anti–CTLA-4, anti–transforming growth factor, or anti–IL-10 antibody. However, the refractory state of CD4 CD25 T cells was partially reversible by the addition of IL-2 or IL-4. These data demonstrate that human blood contains a resident T cell population with potent regulatory properties.