In normal mice, a subpopulation of CD4 T cells constitutively express CD25. These cells behave as regulatory T cells in autoimmune and inflammatory reactions, in tolerance to superantigens, and in peripheral T-cell homeostasis. They are unable to produce interleukin (IL)-2, and are dependent on IL-2 for growth in vitro and in vivo. CD4 CD25(+) T cells spontaneously secrete IL-10, which is involved in some of their regulatory functions. They are resistant to apoptosis, but can be tolerized by anergy.