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.