T-cell alloreactivity directed against non–self-HLA molecules has been assumed to be less peptide specific than conventional T-cell reactivity. A large variation in degree of peptide specificity has previously been reported, including single peptide specificity, polyspecificity, and peptide degeneracy. Peptide polyspecificity was illustrated using synthetic peptide-loaded target cells, but in the absence of confirmation against endogenously processed peptides this may represent low-avidity T-cell reactivity. Peptide degeneracy was concluded based on recognition of Ag-processing defective cells. In addition, because most investigated alloreactive T cells were in vitro activated and expanded, the previously determined specificities may have not been representative for alloreactivity in vivo. To study the biologically relevant peptide specificity and avidity of alloreactivity, we investigated the degree of peptide specificity of 50 different allo-HLA–reactive T-cell clones which were activated and expanded in vivo during GVHD. All but one of the alloreactive T-cell clones, including those reactive against Ag-processing defective T2 cells, recognized a single peptide allo-HLA complex, unique for each clone. Down-regulation of the expression of the recognized Ags using silencing shRNAs confirmed single peptide specificity. Based on these results, we conclude that biologically relevant alloreactivity selected during in vivo immune response is peptide specific.