Regulatory T (T_R) cells manifest constitutive expression of cytotoxic T lymphocyte–associated antigen 4 (CTLA-4), but the function of CTLA-4 in mediating the regulatory function of T_R cells is unclear. We show here that mouse CD4⁺CD25⁺ cells, either resting or induced to overexpress CTLA-4 by treatment with antibody to CD3, initiated tryptophan catabolism in dendritic cells through a CTLA-4-dependent mechanism. This process required B7 expression and cytokine production by the dendritic cells. In contrast, T_R cells cultured in the presence of bacterial lipopolysaccharide induced tryptophan catabolism by dendritic cells in a CTLA-4-independent but cytokine-dependent way. Thus, regulation of immunosuppressive tryptophan catabolism in dendritic cells might represent a major mechanism of action of T_R cells.