Alloantibodies can play a key role in acute and chronic allograft rejection. However, relatively little is known of factors that control B cell responses following allograft tolerance induction. Using 3-83 Igi mice expressing an alloreactive BCR, we recently reported that allograft tolerance was associated with the sustained deletion of the alloreactive B cells at the mature, but not the immature, stage. We have now investigated the basis for the long-term control of alloreactive B cell responses in a non-BCR-transgenic model of C57BL/6 cardiac transplantation into BALB/c recipients treated with anti-CD154 and transfusion of donor-specific spleen cells. We demonstrate that the long-term production of alloreactive Abs by alloreactive B cells is actively regulated in tolerant BALB/c mice through the dominant suppression of T cell help. Deletion of CD25(+) cells resulted in a loss of tolerance and an acquisition of the ability to acutely reject allografts. In contrast, the restoration of alloantibody responses required both the deletion of CD25(+) cells and the reconstitution of alloreactive B cells. Collectively, these data suggest that alloreactive B cell responses in this model of tolerance are controlled by dominant suppression of T cell help as well as the deletion of alloreactive B cells in the periphery.