Unresponsiveness towards autoantigens or potentially harmful pathogens can be thought of as an active process of developing tolerance. The induction of this activity relies on the interaction of antigen-presenting cells (APCs) and T cells, causing T-cell tolerance. Very recently, a new feature of regulatory APCs was observed. The expression of indoleamine-2,3-dioxygenase (IDO) activity by certain dendritic cells (DCs), monocytes and macrophages has immunomodulatory effects on T cells that are related to the peri-cellular degradation of the essential amino acid tryptophan [ 1 ]. In their recent review, Grohmann and co-authors [ 2 ] described the central role of IDO in the control of T-cell activity during infection, pregnancy, autoimmunity, transplantation and neoplasia.