P ancreatic islet transplantation represents a potential treatment for insulin-dependent diabetes mellitus. However, the precise cellular and molecular mechanisms of the immune reactions against allogeneic and xenogeneic transplanted islets remain unclear. Here, we demonstrate that CD4⁺ Vα14 natural killer T (NKT) cells, a recently identified lymphoid cell lineage, are required for the acceptance of intrahepatic rat islet xenografts. An anti-CD4 mAb, administrated after transplantation, allowed islet xenografts to be accepted by C57BL/6 mice, with no need for immunosuppressive drugs. The dose of anti-CD4 mAb was critical, and the beneficial effect appeared to be associated with the reappearance of CD4⁺ NKT cells at around 14 days after transplantation. Interestingly, rat islet xenografts were rejected, despite the anti-CD4 mAb treatment, in Vα14 NKT cell–deficient mice, which exhibit the normal complement of conventional lymphoid cells; adoptive transfer of Vα14 NKT cells into Vα14 NKT cell–deficient mice restored the acceptance of rat islet xenografts. In addition, rat islet xenografts were accepted by Vα14 NKT mice having only Vα14 NKT cells and no other lymphoid cells. These results indicate that Vα14 NKT cells play a crucial role in the acceptance of rat islet xenografts in mice treated with anti-CD4 antibody, probably by serving as immunosuppressive regulatory cells.