Using an in vitro peptide stimulation strategy, two chimpanzees that were acutely infected by the hepatitis B virus (HBV) produced peripheral blood CTL responses to several HBV-encoded epitopes that are known to be recognized by class I-restricted CTL in acutely infected humans. One animal responded to three HBV peptides that, in humans, are restricted by HLA-A2; the other animal responded to three peptides that are restricted by HLA-B35 and HLA-B51, members of the HLA-B7 supertype in man. The peptides recognized by each chimp corresponded with the ability of its class I molecules to bind peptides containing the HLA-A2 and HLA-B7 supermotifs. Similar, apparently class I-restricted CTL responses to some of these peptides were also detected in occasional HBV-uninfected chimps. These results demonstrate that the CTL repertoire overlaps in humans and chimps and that the HLA-A2 and HLA-B7 supertypes extend to the chimpanzee. Based on these results, the immunogenicity and efficacy of vaccines designed to induce CTL responses to human HLA-restricted viral epitopes may be testable in chimpanzees.