An Epstein-Barr virus (EBV)-specific CD8+ cytotoxic T lymphocyte (CTL) clone (LC13) was shown to recognize the minimal peptide determinant FLRGRAYGL from the EBNA 3 antigen of the BL74 strain of EBV. The equivalent epitope from the B95-8 strain (FLRGRAYGI) is not recognized when endogenously presented and the peptide is 15-fold less active than FLRGRAYGL. A replacement set of peptides was synthesized in which each residue within FLRGRAYGL was sequentially replaced with all other genetically coded amino acids. These peptides were tested for their ability to sensitize target cells to lysis by LC13. Of the 171 single-amino acid replacement peptides only 15 were more active than the peptide FLRGRAYGI. Five peptides had significantly greater activity than FLRGRAYGL and a peptide incorporating the most active of these single-amino acid substitutions (HIRGRAYSL) induced lysis at concentrations approximately 30-fold less than FLRGRAYGL. Simplified theoretical calculations based on this study suggest that CTL LC13 has a specificity for its target epitope of 1 in 4.7 x 10(10). This represents the first complete analysis of the role of single amino acids within a minimum epitope on the specificity of CTL recognition.