Following allotransplantation, determinants encoded within the donor MHC are recognized by recipient T lymphocytes through their Ag receptor. In this study, we investigated the TCR Vbeta chain diversity of T cells infiltrating rejected and tolerated heart allografts in a model of donor-specific blood transfusion-induced tolerance in MHC-mismatched congeneic rats. The PCR-based method that we used allows the diversity of Vbeta chains at the complementarity-determining region 3 level to be analyzed quantitatively. Our results show that the Vbeta repertoire usage in graft-infiltrating T cells was characteristic and different in tolerated compared with rejected grafts, and differed in both cases from the normal distribution of the Vbeta repertoire. An expansion of lymphocytes showing a conserved Vbeta18-Dbetal-Jbeta2.7 gene rearrangement was found, from the first day after grafting onward, in graft-infiltrating cells from all tolerant animals. This clone accounted for as much as 5% of the whole Vbeta repertoire in tolerated hearts, as evidenced by RNase protection assay. In contrast, we demonstrated that, of lymphocytes infiltrating rejected grafts, those with a Vbeta18 chain were diverse, and that even though by day 5 the conserved Vbeta18-Dbeta1-Jbeta2.7 rearrangement was detectable, lymphocytes harboring this rearrangement represented less than 0.6% of the whole TCR-alphabeta+ T cell repertoire. Kinetics analysis revealed that the expansion of lymphocytes bearing this conserved rearrangement was elicited specifically by donor blood transfusion. Indeed, Vbeta18-Dbeta1-Jbeta2.7 transcripts were detected in PBL from transfused animals as early as 7 days after donor-specific blood transfusion. Finally, we provided evidence that this T cell clone belongs to the CD8+ subset. The putative role in inducing and maintaining the allograft tolerance of the CD8+ T cell clone harboring this public Vbeta18-Dbeta1-Jbeta2.7 rearrangement is discussed.