The crystal structure of the HLA-Aw68. 1 antigen binding site revealed a negatively charged pocket centred on aspartic acid 74 (Garrett et al. 1989). Access to this `74 pocket' is blocked in HLA-Aw68. 2 and HLA-Aw69 by two substitutions at positions 97 and 116. This key feature suggests that the Aw68. 1-peptide-specific interactions may involve salt bridges between oppositely charged residues. In this paper, the influenza epitope recognized by virus-specific HLA-Aw68. 1-restricted cytotoxic T lymphocytes (CTL) has been defined in vitro with a synthetic peptide corresponding to residues 89-101 of the nucleoprotein (NP). Amino acid substitutions of the peptide NP 89-101 showed that the arginine at position 99 is an anchor point of the peptide within the Aw68. 1 antigen binding site. Consistent with this we find that neither HLA-Aw68. 2 nor HLA-Aw69 positive cells can present peptide NP 89-101 to Aw68. 1-restricted CTL. Our results therefore suggest a model in which presentation of NP 89-101 by HLA-Aw68. 1 is dependent upon interaction of the positively charged arginine residue at position 99 of the peptide, with the negatively charged aspartic acid in the `74 pocket' of HLA-Aw68. 1. We also show that influenza-virus-specific HLA-Aw68. 1-restricted CTL are CD8 independent. This result is consistent with the low affinity of HLA-Aw68. 1 for CD8 (Salter et al. 1989) and reveals a unique example of CD8-independent priming of CTL by natural infection with a common pathogen in humans.