Two distinct types of CpG oligodeoxynucleotide (ODN) have been identified that differ in their capacity to stimulate antigen-presenting cells: CpG-A induces high amounts of interferon-α (IFN-α) and IFN-β in plasmacytoid dendritic cells (PDCs), whereas CpG-B induces PDC maturation and is a potent activator of B cells but stimulates only small amounts of IFN-α and IFN-β. Here we examined the ability of these CpG ODNs to enhance peptide-specific CD8⁺ T-cell responses in human peripheral blood mononuclear cells (PBMCs). The frequency of influenza matrix–specific “memory” CD8⁺ T cells was increased by both types of CpG ODN, whereas the frequency of Melan-A specific “naive” CD8⁺ T cells increased on stimulation with CpG-B but not with CpG-A. The presence of PDCs in PBMCs was required for this CpG ODN-mediated effect. The expanded cells were cytotoxic and produced IFN-γ on peptide restimulation. Soluble factors induced by CpG-A but not CpG-B increased the granzyme-B content and cytotoxicity of established CD8⁺ T-cell clones, each of which was IFN-α/-β dependent. In conclusion, CpG-B seems to be superior for priming CD8⁺ T-cell responses, and CpG-A selectively enhances memory CD8⁺ T-cell responses and induces cytotoxicity. These results demonstrate distinct functional properties of CpG-A and CpG-B with regard to CD8 T cells.