T-CELL differentiation in the thymus is thought to involve a progression from the CD4⁻CD8⁻ phenotype through CD4⁺CD8⁺ intermediates to mature CD4⁺ or CD8⁺ cells^1-3. There is evidence that during this process T cells bearing receptors potentially reactive to 'self are deleted by a process termed 'negative selection'^4-10. One example of this process occurs in mice carrying polymorphic Mis antigens, against which a detectable proportion of T cells are autoreactive. These mice show clonal deletion of thymic and peripheral T-cell subsets that express the autoreactive V_β3 segment of the T-cell antigen receptor, but at most a two-fold depletion of thymic cells at the CD4⁺CD8⁺ stage^5-7. By contrast, transgenic mice bearing both α and β chain genes encoding autoreactive receptors recognizing other ligands, show severe depletion of CD4⁺CD8⁺ thymocytes as well^8,9, suggesting that negative selection occurs much earlier. We report here the Mis 2^a/3^a mediated elimination of T cells expressing a transgene encoded V_β3-segment, in T-cell receptor α/β and β-transgenic mice. Severe depletion of CD4⁺CD8⁺ thymocytes is seen only in the α/β chain transgenic mice, whereas both strains delete mature V_β3 bearing CD4⁺ and CD8⁺ T cells efficiently. We conclude that severe CD4⁺CD⁸ thymocyte deletion in α/β transgenic mice results from the premature expression of both receptor chains, and does not reflect a difference in the timing or mechanism of negative selection for Mis antigens^5-7 as against the allo- and MHC class 1-restricted antigens used in the other studies^8-9.