In previous studies, the antigen CSH-275 (RTNKEASIC) was found expressed in tissue specimens from colorectal cancer but not in normal colonic mucosa. It was also naturally expressed in the DHD-K12 experimental colorectal cancer in BDIX rats. In this study, we describe the effect of vaccination with the synthetic nonapeptide CSH-275 in preventing tumor growth in a model closely mimicking the clinical situation of liver metastases, after surgical resection of primary colorectal cancer. A vaccination protocol using CSH-275, conjugated with complete or incomplete Freund's adjuvant, was carried out to determine the effect in preventing the progression of liver metastases induced by DHD-K12 cells injected in the splenic vein (preventive vaccine). An additional vaccination procedure was carried out to determine the effect on s.c. tumor growth (therapeutic vaccine). A significant improvement in survival along with the prevention of liver metastases formation and reduced growth of s.c. tumor were observed. CSH-275 vaccination resulted in a significant increase in CTL activity against autologous DHD-K12 cells in DHD-K12 tumor-bearing rats and the generation of a CTL response against DHD-K12 cells in DHD-K12 naive rats. Vaccination also induced massive infiltration of CD8+ cells in tumor. These results demonstrate that CSH-275 is a new molecular target for colorectal cancer immunotherapy; it is also an excellent candidate for preclinical studies because it is naturally expressed on tumors in a fully competent syngeneic animal, which reproduces the clinical pattern of cancer progression. © 2004 Wiley-Liss, Inc.