CLONAL deletion in the thymus plays a major part in T-cell tolerance to self antigens^1–3. But the mechanism of negative selection, its fine specificity and the threshold of affinity and avidity remains unknown. We have now examined these aspects of negative selection with mice expressing a transgenic T-cell receptor with specificity for lymphocytic choriomeningitis virus (LCMV) glycoprotein in association with the class I H–2D^b molecule. These mice were rendered tolerant to LCMV by neonatal infection with mutant LCMVs hearing point mutations in the T-cell epitope recognized by the transgenic T-cell receptor (ref. 4). Variant LCMVs were also tested for their ability to elicit antiviral responses in transgenic mice in vivo and in vitro. Comparison in vivo revealed that a low-avidity receptor interaction, which was unable to induce effector T cells in the periphery, was still sufficient for clonal deletion in the thymus.