TCRα-and β-chains cooperatively recognize peptide–MHC complexes. It has been shown that a “chain-centric” TCR hemichain can, by itself, dictate MHC-restricted Ag specificity without requiring major contributions from the paired TCR counterchain. Little is known, however, regarding the relative contributions and roles of chain-centric and its counter, non–chain-centric, hemichains in determining T cell avidity. We comprehensively analyzed a thymically unselected T cell repertoire generated by transducing the α-chain–centric HLA-A* 02: 01 (A2)/MART1 27–35 TCRα, clone SIG35α, into A2-matched and unmatched postthymic T cells. Regardless of their HLA-A2 positivity, a substantial subset of peripheral T cells transduced with SIG35α gained reactivity for A2/MART1 27–35. Although the generated A2/MART1 27–35–specific T cells used various TRBV genes, TRBV27 predominated with> 10 2 highly diverse and unique clonotypic CDR3β sequences. T cells individually reconstituted with various A2/MART1 27–35 TRBV27 TCRβ genes along with SIG35α possessed a wide range (> 2 log orders) of avidity. Approximately half possessed avidity higher than T cells expressing clone DMF5, a naturally occurring A2/MART1 27–35 TCR with one of the highest affinities. Importantly, similar findings were recapitulated with other self-Ags. Our results indicate that, although a chain-centric TCR hemichain determines Ag specificity, the paired counterchain can regulate avidity over a broad range (> 2 log orders) without compromising Ag specificity. TCR chain centricity can be exploited to generate a thymically unselected Ag-specific T cell repertoire, which can be used to isolate high-avidity antitumor T cells and their uniquely encoded TCRs rarely found in the periphery because of tolerance.