Previous studies have indicated that most HLA-A2-binding peptides are 9 amino acid (aa) residues long, with a Leu at position 2 (P2), and a Val or Leu at P9. We compared the binding properties of different peptides by measuring the rate of dissociation of beta 2-microglobulin from peptide-specific HLA-A2 complexes. The simplest peptide that we identified that could form HLA-A2 complexes had the sequence (in single letter aa code) GLFGGGGGV, indicating that three nonglycine aa are sufficient for binding to HLA-A2. To determine whether most nonapeptides that contained Leu at P2 and Val or Leu at P9 could bind to HLA-A2, we tested the binding of nonapeptides selected from published HIV and melanoma protein sequences, and found that six of seven tested formed stable HLA-A2 complexes. We identified an optimal antigenic undecapeptide from the cytomegalovirus gB protein that could form stable HLA-A2 complexes that contained apparent anchor residues at P2 and P11 (sequence FIAGN-SAYEYV), indicating that the spacing between anchor residues can be somewhat variable. Finally, we tested the importance of every aa in the influenza A matrix peptide 58-66 (sequence GILGFVFTL) for binding to HLA-A2, by using Ala-substituted and Lys-substituted peptides. We found that multiple positions were important for stable binding, including P2, P3, P5-P7, and P9. We conclude that the P2 and P9 anchor residues are of prime importance for peptide binding to HLA-A2. However, other peptide side chains (especially at P3) contribute to the stability of the interaction. In certain cases, the optimal length for peptide binding can be longer than 9 residues.