CD4+CD25+ T cells have been shown to inhibit experimentally induced organ-specific autoimmune disease and depletion of these regulatory T cells from normal mice results in development of such conditions. Furthermore, CD4+CD25+ T cells suppress the IL-2 production and thereby the proliferation of polyclonally activated CD4+CD25– T cells in vitro. The suppression in vitro is independent of secreted factors but requires interactions between CD4+CD25– and CD4+CD25+ T cells and antigen-presenting cells (APC). We have now further investigated the function of CD4+CD25+ T cells in vitro and have focused on their interactions with APC. We found that CD4+CD25+ T cells down-regulated the expression of the co-stimulatory molecules CD80 and CD86 on dendritic cells. The steady-state level of CD80 mRNA was also decreased, while the steady-state level of CD86 mRNA was not, suggesting that distinct mechanisms regulate the expression of these molecules. The down-regulation occurred even in the presence of stimuli that would normally increase the expression of CD80 and CD86 molecules. Thus, down-regulation of co-stimulatory molecules may be an additional effector function of these regulatory T cells.