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T cells compete in the response to antigen in vivo and this competition may drive the affinity maturation of a secondary T cell response. Here we show that high-affinity T cells out-competed lower affinity T cells during a response to antigenic challenge in vivo. Although competition between T cells specific for different peptide–major histocompatibility complexes (MHC) occurred, it was less efficient than competition between T cells of the same peptide-MHC specificity. In addition, high-affinity T cells efficiently induced antigen loss from the surface of antigen-presenting cells. Thus T cells that responded to the same peptide-MHC competed with each other by lowering the amount of ligand with which the cells could react. As a result, the activation of high-affinity cells was favored. This provides a mechanism for the affinity maturation of a secondary T cell response.