We previously reported acid-extracted natural antigenic peptide (F4.2 [YSWMDISCWI]) of a gastric signet ring cell carcinoma HST-2 cells, recognized by HLA-A*31012-restricted autologous cytotoxic T lymphocytes, TcHST-2 line. In this study, the full-length cDNA (1101 bp), termed c98, predicting a protein composed of 170 amino acids was obtained. Because TcHST-2 cells could lyse the HLA-A31 antigen (+) allogeneic tumor cells that were introduced with c98 gene, this gene was suggested to possess antigenicity. Beginning at N-terminal 61 amino acid, the N-terminal six amino acid sequence that is completely identical to F4.2 was present in c98; however, a sequence of four amino acids in C-terminal was not found. Nevertheless, this peptide, c9861–70, seemed to be immunogenic, because cells pulsed with c9861–70peptide were lysed in a dose-dependent manner by TcHST-2 cells. The c98 gene was expressed ubiquitously in tumor cells as well as in normal tissues. However, some tumor cells, including HST-2 cells, expressed this antigen in a high content, and such cells were lysed by TcHST-2 cells in the context of HLA-A31 antigen. However, TcHST-2 cells did not lyse cells that expressed lower amounts of c98 than HST-2 cells. These data suggested that c98-gene product and/or c9861–70peptides could be used as a candidate for tumor vaccines in cancer immunotherapy.