A dministration of attenuated, activated autoimmune T lymphocytes to syngeneic mice and rats has been shown to prevent or induce remission of experimental autoimmune diseases specific for the autoimmune T cells. The process has been termed "T cell vaccination." In a recent study, T cell vaccination was done using T cells sensitized to rat alloantigens. The procedure produced a significant reduction of the mixed lymphocyte reaction (MLR) against allogeneic cells. The reduction in MLR was not specific: Vaccination with T cells specific for stimulator cells of one allotype led to a reduced MLR stimulated by cells of another allotype. The present study was undertaken to examine whether T cell vaccination can induce tolerance to transplantation antigens in vivo. We used the model of heterotopic cardiac transplantation in rats. We now report that vaccinating rats with syngeneic, activated, alloantigen-primed T lymphocytes significantly prolonged survival of rat cardiac allografts. The effect of T cell vaccination was most evident when the T cells had been obtained from rats specifically sensitized against the donor rats: Brown-Norway (BN) allografts in control Wistar rats survived 8.5 +/- 0.4 d while BN allografts survived 29.2 +/- 7.1 d in Wistar rats that had been vaccinated with Wistar anti-BN cells. Vaccination of Wistar rats with Wistar anti-hooded T cells prolonged survival of BN heart allografts to a lesser but significant degree (13.0 +/- 1.1 d). Thus, T cell vaccination of recipients can prolong survival of allografts.