In the clinical setting, transplanted liver seems to protect other grafts from the same donor from rejection. Our previous findings suggest that an auxiliary liver transplantation a few hours before a renal transplantation not only inhibits hyperacute antibody-mediated rejection but also improves long-term kidney graft survival in sensitized recipients. Here, we investigated indoleamine 2,3-dioxygenase (IDO) activity, as one potential mechanism for liver-induced long-term acceptance of kidney grafts.