Allograft rejection is a predominantly Th1 immune response. In this study, we showed that obaculactone, a natural compound derived from citrus fruit, prolonged skin graft survival in mice when treated after but not before transplantation. Furthermore, obaculactone inhibited alloantigenspecific production of Th1 cytokine IFN-g as well as proinflammatory cytokine IL-2, TNFa and IL-6. In parallel, IL-10 production was markedly up-regulated. Obaculactone significantly enhanced the percentage of CD4+ CD25+ Foxp3+ Treg cells in the CD4+ splenocytes without any effect on their inhibitory function. In vitro and in vivo tests showed obaculactone down-regulated T-bet expression in Th1 effector cells. Taken together, the unique immunomodulatory properties might qualify obaculactone as a putative, therapeutic compound for the treatment of Th1-driven diseases, including transplant rejection.