The detection and monitoring of peptide-specific cytotoxic T lymphocyte (CTL) precursors is essential for successful peptide-based immunotherapy against cancers. In contrast to the development of effective methods of detecting antigen-specific CTL, such as ELISpot and HLA-class I tetramer assay, stimulation with peptide-pulsed antigen-presenting cells (APC) has for some time been conventionally employed to induce peptide-specific CTL from peripheral blood mononuclear cells (PBMC). This culture protocol, however, needs a substantial number of PBMC to test the reactivity against a panel of peptides. In the present study, we established a simple culture protocol which has no need of additional APC. Addition of a corresponding peptide every 3 days was found to induce not only Epstein–Barr virus (EBV)-specific CTL from healthy donors, but also tumor antigen-derived peptide-specific CTL from cancer patients. A 10-ml blood sample was almost sufficient to test the presence of CTL precursors against 20 different peptides in triplicate assays. Overall, this culture protocol can be useful in detecting and monitoring peptide-specific CTL precursors in the circulation in peptide-based immunotherapy against cancer.