DEVELOPMENT of thymocytes involves two distinct outcomes resulting from superficially similar events. Recognition by thymocytes of major histocompatibility complex (MHC) proteins plus pep-tide leads to their rescue from apoptosis (positive selection), and recognition of antigenic peptide induces cell death (negative selection)¹. Antigen analogues^1–3, and sometimes low concentrations of antigenic peptide^4,5, induce positive selection; such analogues are often antagonists of mature T-cell clones^1–3,6. Various models seek to explain how recognition of different peptide/MHC complexes leads to such different outcomes^1,7–10: quantitative models relate response to the affinity, avidity or kinetics of T-cell-antigen receptor (TCR) binding, whereas qualitative models require conformational or spatial changes in the TCR or associated molecules to modulate signal transduction^7,9. We have used surface plasmon resonance¹¹ to measure the kinetics of TCR interactions with positively and negatively selecting ligands to distinguish between these models, and find that affinity correlates to the outcome of selection. A 'window' of affinity resulting in positive selection extends over a 1-log range starting threefold below the affinity for negative selection.