Clonal deletion of self antigen-reactive T lymphocytes is known to be a dominant mechanism of tolerance induction in the normal immune system. This report considers whether deletion of antigen-reactive T cells is also the immunologic basis for the recently described model of transplantation tolerance that follows intra-thymic inoculation with allogeneic lymphoid cells. We found that the outcome of injecting Mls a-hosts with lymphocytes from Mls a+ donors was depletion of Vβ6+ T cells (which are known to be reactive with the Mls a superantigen). The process was found to be specific in that a similar reduction was not seen in an irrelevant T cell population (Vβ8+) in IT injected hosts. Deletion was observed in this model only if immunosuppression with ALS or anti-CD4 accompanied intrathymic injection. When the inoculum of allogeneic lymphocytes was administered intravenously instead of intrathymically only minimal deletion was observed. The induction of transplantation tolerance by intrathymic injection of donor lymphoid cells may prove especially efficacious since it relies on deletion of only those T cells specifically reactive to donor antigens, a process analogous to tolerance induction to self antigens.