Donor-specific allograft tolerance can be induced in adult rats by pregraft donor-specific blood transfusion (DST). We have previously shown that DST elicits in recipients the expansion of a donor-specific CD8+ T cell clone displaying the Vbeta18-Dbeta1-Jbeta2.7 TCR rearrangement, which rapidly infiltrates allografts after transplantation, suggesting a regulatory function for this clone in DST-induced tolerance. In this study, recipients were pretreated before grafting, using an anti-CD8 monoclonal antibody to deplete CD8+ T cells. CD8 depletion before DST and transplantation abrogated allograft tolerance, and the CD8+ T cell clone was absent from allografts. These effects were not observed when CD8 depletion was performed after DST but before transplantation. We conclude that CD8+ T cells play a role in the induction of allograft tolerance by DST.