To escape immune recognition, viruses acquire amino acid substitutions in class I human leukocyte antigen (HLA)-presented cytotoxic T-lymphocyte (CTL) epitopes. Such viral escape mutations may (i) prevent peptide processing, (ii) diminish class I HLA binding, or (iii) alter T-cell recognition. Because residues 418 to 426 of the hypervariable influenza A virus nucleoprotein (NP(418-426)) epitope are consistently bound by class I HLA and presented to CTL, we assessed the impact that intraepitope sequence variability has upon T-cell recognition. CTL elicited by intranasal influenza virus infection were tested for their cross-recognition of 20 natural NP(418-426) epitope variants. Six of the variant epitopes, of both H1N1 and H3N2 origin, were cross-recognized by CTL while the remaining NP(418-426) epitope variants escaped targeting. A pattern emerged whereby variability at position 5 (P5) within the epitope reduced T-cell recognition, changes at P4 or P6 enabled CTL escape, and a mutation at P8 enhanced T-cell recognition. These data demonstrate that substitutions at P4 and/or P6 facilitate influenza virus escape from T-cell recognition and provide a model for the number, nature, and location of viral mutations that influence T-cell cross-recognition.