Signals generated through CD28–B7 and CD40 ligand (CD40L)–CD40 interactions have been shown to be crucial for the induction of long-term allograft survivability. We have recently demonstrated that humanized anti-CD40L (hu5C8) prevents rejection of mismatched renal allografts in primates. To investigate potential mechanisms of CD40L–induced allograft acceptance, we coimmobilized hu5C8 with suboptimal amounts of anti-CD3 to stimulate CD4⁺ T cells. We now report that anti-CD3/CD40L costimulation results in CD28-independent activation and subsequent deletion of resting T cells. Coligation of CD3 and CD40L increased expression of CD69, CD25, and CD54 on CD4⁺ T cells. We also found that costimulation with anti-CD3/CD40L resulted in enhanced production of interleukin (IL)-10, interferon γ, and tumor necrosis factor α but not IL-2 or IL-6. Interestingly, after several days, anti-CD3/CD40L–mediated activation was followed by apoptosis in a significant population of cells. Consistent with that observation, anti-CD3/CD40L did not enhance the antiapoptotic proteins Bcl-2 and Bcl-xL. Further, the addition of CD28 at 24 h failed to rescue those cells induced to die after costimulation with anti-CD3/CD40L. Together, these data suggest that the graft-sparing effect of hu5C8 in vivo may result in part from early and direct effects on CD4⁺ T cells, including a vigorous induction of immunomodulatory cytokines and/or apoptosis of allograft-specific T cells.