THE crucial role of the thymus in immunological tolerance^1–5 has been demonstrated by establishing that T cells are positively selected to express a specificity for self major histocompatibility complex (MHC)^6–8, and that those T cells bearing receptors potentially reactive to self antigen fragments, presumably presented by thymic MHC, are selected against^9–11. The precise mechanism by which tolerance is induced and the stage of T-cell development at which it occurs are not known. We have now studied T-cell tolerance in transgenic mice expressing a T-cell receptor with double specificities for lymphocytic choriomeningitis virus (LCMV)-H-2D^b and for the mixed-lymphocyte stimulatory (Mls^a) antigen. We report that αβTCR transgenic mice tolerant to LCMV have drastically reduced numbers of CD4⁺CD8⁺ thymocytes and of peripheral T cells carrying the CD8 antigen. By contrast, tolerance to Mls^a antigen in the same αβTCR transgenic Mls^a mice leads to deletion of only mature thymocytes and peripheral T cells and does not affect CD4⁺CD8⁺ thymocytes. Thus the same transgenic TCR-expressing T cells may be tolerized at different stages of their maturation and at different locations in the thymus depending on the antigen involved.