B7-1, a co-stimulatory factor, has been reported to induce cytotoxic T lymphocytes (CTL). In the present study, we transfected B7-1 genes into a gastric cancer cell line (2MD3) and analyzed the effects of B7-1 transduction on peritoneal metastasis in vitro and in vivo. We revealed that mononuclear lymphocytes show significantly stronger adherence and cytotoxicity to B7-1 transfected cells (2MD3/B7) than to their parent 2MD3 cells. We also demonstrated that mice inoculated with 2MD3/B7 cells in the peritoneal cavity have a significantly better survival rate than those inoculated with 2MD3 cells (log-rank test, p<0.01). Histologic findings showed that leukocytes intensively infiltrate the 2MD3/B7 metastatic nodules, but can scarcely be observed in the nodules associated with 2MD3 cells. These findings indicate that the B7-1 may play an important role in suppressing peritoneal metastasis by the mechanism of enhanced immunogenicity, and that B7-1 gene transduction might be effective against peritoneal metastases of gastric cancer.