Organ graft rejection is a T cell-dependent process in which activation of alloreactive T cells via the T cell receptor/CD3 complex is a critical step. Although treatment with anti-CD3 has been shown to prevent and reverse allograft rejection, there is little information available regarding the effects of immunotherapy using anti-TCR alpha beta mAb for rejection. In the present study, short-term preoperative treatment of rats with a mAb against alpha beta TCR (R73) completely prevented the rejection of cardiac allografts. These rats accepted second cardiac allografts from the same donor strain, but not from a third-party strain, without additional treatment. In mixed lymphocyte cultures, T cells from rats that had received cardiac grafts did not respond to donor-strain heart cells, but did respond to donor-strain spleen cells and third-party heart cells. These findings suggest that specific unresponsiveness to cardiac tissue was induced in R73-treated rats. Such unresponsiveness was induced only when rats were pretreated with the mAb and subsequently received a transplant. It is likely that administration of a small dose of R73 induced transient immunomodulation of TCR molecules, resulting in unresponsiveness to a subsequent cardiac allograft. Immunotherapy with mAb against TCR alpha beta is very effective, without apparent side effects, and may provide a new method for preventing graft rejection.