The impact of memory B cells and alloantibodies on the ability to induce transplantation tolerance has not been elucidated. We have developed a murine heart transplant model that isolates the contributions of functional memory B cells from memory T cells in allograft rejection. Memory 3-83 B cells with dual specificity for H-2K k and H-2K b were generated in 3-83 Igi BCR knockin (BALB/c background) mice by the transplantation of C3H (H-2K k) hearts in the absence of immunosuppression. To test the effect of functional memory 3-83 B cells, C3H-primed 3-83 Igi recipients were challenged with C57BL/6 hearts (H-2K b) at 60–90 days post-C3H heart transplant and treated with anti-CD154 mAbs. Despite immunosuppression, the C57BL/6 hearts were acutely rejected within 10–13 days and graft rejection was associated with increased frequencies of C57BL/6-specific IFN-γ-producing T cells. Histology revealed significant numbers of infiltrating T cells, consistent with acute T cell-mediated rejection. The resistance to tolerance induction was dependent on the synergistic effects of memory 3-83 B cells and alloantibodies, whereas memory T cells are not necessary. We conclude that the combined effects of functional memory B cells and alloantibodies prevent anti-CD154-mediated graft acceptance by facilitating the CD40-CD154-independent activation of alloreactive T cells. This study provides insight into the potential ability of memory B cells and alloantibodies to prevent anti-CD154-mediated graft acceptance.