The molecular basis underlying the specificity of alloreactive T cells for peptide–major histocompatibility complex ligands has been elusive. Here we describe a screen of 60 I-E^k-alloreactive T cells and 83 naturally processed peptides that identified 9 reactive T cells. Three of the T cells responded to multiple, distinct peptides that shared no sequence homology. These T cells recognized each peptide–major histocompatibility complex ligand specifically and used a distinct constellation of I-E^k contact residues for each interaction. Our studies show that alloreactive T cells have a 'germline-encoded' capacity to recognize multiple, distinct ligands and thus show 'polyspecificity', not degeneracy. Our findings help to explain the high frequency of alloreactive T cells and provide insight into the nature of T cell specificity.