The promotion of alloengraftment in the absence of global immune suppression and multiorgan toxicity is a major goal of transplantation. It is demonstrated that the infusion of a single modest bone marrow dosage in 200 cGy-irradiated recipients treated with anti-CD154 (anti-CD40L) monoclonal antibody (mAb) resulted in chimerism levels of 48%. Reducing irradiation to 100 or 50 cGy permitted 24% and 10% chimerism, respectively. In contrast, pan–T-cell depletion resulted in only transient engraftment in 200 cGy-irradiated recipients. Host CD4⁺ cells were essential for alloengraftment as depletion of CD4⁺ cells abrogated engraftment in anti-CD154–treated recipients. Strikingly, the depletion of CD8⁺ cells did not further enhance engraftment in anti-CD154 mAb–treated recipients in a model in which rejection is mediated by both CD4⁺ and CD8⁺ T cells. However, anti-CD154 mAb did facilitate engraftment in a model in which only CD8⁺ T cells mediate rejection. Furthermore, CD154 deletional mice irradiated with 200 cGy irradiation were not tolerant of grafts, suggesting that engraftment promotion by anti-CD154 mAb may not simply be the result of CD154:CD40 blockade. Together, these data suggest that a CD4⁺regulatory T cell may be induced by anti-CD154 mAb. In contrast to anti-CD154 mAb, anti-B7 mAb did not promote donor engraftment. Additionally, the administration of either anti-CD28 mAb or anti-CD152 (anti–CTLA-4) mAb or the use of CD28 deletional recipients abrogated engraftment in anti-CD154 mAb–treated mice, suggesting that balanced CD28/CD152:B7 interactions are required for the engraftment-promoting capacity of anti-CD154 mAb. These data have important ramifications for the design of clinical nonmyeloablative regimens based on anti-CD154 mAb administration.