After exposure to alloantigen in vivo and in vitro, alloantigen reactive immunoregulatory activity is enriched in a population of CD4⁺ T cells that express high levels of CD25.  In vivo, common mechanisms underpin the activity of CD25⁺CD4⁺ Treg in both naive and manipulated adult hosts.   We have identified a unique role for IFNg in the functional activity of CD25⁺CD4⁺ alloantigen reactive Treg during the development of operational tolerance to donor alloantigens in vivo that is consistent with observations showing that tolerance to alloantigens cannot be induced in the absence of IFNg. The identification and characterisation of Treg that can control immune responsiveness to alloantigens has opened up exciting opportunities for new therapies in transplantation.  These observations may have important implications for the design of clinical protocols to induce allograft tolerance in adult recipients.