Pregnancy is a natural model of successful tolerance induction against allogeneic tissues. Recent studies pointed to a role of indoleamine 2,3-dioxygenase (IDO), a tryptophan-degrading enzyme expressed in the placenta, in mediation of T-cell suppression. We want to apply to organ transplantation what nature has developed for suppression of fetal rejection during pregnancy. Here we analyze whether IDO-induced tryptophan metabolites are able to suppress the allogeneic T-cell response and allograft rejection in rats. Rat lymphocytes were stimulated with allogeneic dendritic cells in vitro in the presence of increasing amounts of tryptophan metabolites (kynurenine, 3-hydroxykynurenine, anthranilic acid, 3-hydroxyanthranilic acid and quinolinic acid) and T-cell proliferation was determined. The findings showed that kynurenine, 3-hydroxykynurenine and 3-hydroxyanthranilic acid strongly suppress the T-cell response, whereas anthranilic and quinolinic acid are non-effective. Vital staining of cells with subsequent fluorescence-activated cell sorter analyses demonstrated that suppression is mediated by T-cell death. Thereafter, the action of metabolites was analyzed in a skin allograft model (BN-->LEW). Lewis recipients received daily s.c. injections of tryptophan metabolite mixture (kynurenine + 3-hydroxyanthranilic acid), cyclosporin A (positive control), or no treatment (negative control). The metabolites induced a significant prolongation (P = 0.0018) of graft survival. We conclude that IDO-induced tryptophan metabolites suppress the T-cell response and prolong allograft survival in rats.