We tried to identify prostate-specific membrane antigen (PSMA)-derived peptides capable of eliciting both cellular and humoral immune responses in peripheral blood mononuclear cells (PBMCs) and plasma of HLA-A24(+) prostate cancer patients, respectively. For cellular response, peptide-specific and prostate cancer-reactive responses of in vitro-stimulated PBMCs were examined with regard to interferon (IFN)-gamma production and cytotoxicity against both a parental HLA-A24(-) prostate cancer cell line (PC-93) and an HLA-A24-expressing transfectant cell line (PC93-A24). For humoral response, patients' plasma was tested for reactivity to the peptides by means of an enzyme-linked immunosorbent assay (ELISA). Among 13 PSMA peptides, PSMA 624-632 peptide induced peptide-specific and tumor-reactive cytotoxic T lymphocytes (CTLs) most effectively. The PSMA 624-632 peptide-stimulated PBMCs from either healthy donors or prostate cancer patients produced a significant level of IFN-gamma in response to prostate cancer cells in an HLA-A24-restricted manner, and also showed a higher level of cytotoxicity against PC93-A24 than against PC93. Antibodies to the PSMA 624-632 peptide, but not to any others, were detected in prostate cancer patients. These results demonstrate that the PSMA 624-632 peptide could be an appropriate molecule for use in specific immunotherapy of HLA-A24(+) patients with prostate cancer.