AIM To establish a relevant animal model of human gastrointestinal cancer, which can be used forrepetitive investigations and may improve our understanding of carcinogenesis and cancer metastasis.METHODS Intact tissue of human colorectal and pancreatic cancers was transplanted in nude mice. Thebiological characteristics of the original and corresponding transplanted tumors were investigated by HEstaining, PAS staining and immunostaining. The metastases in livers and lungs of the nude mice wereinvestigated by immunostaining with biotinylated mab KL-1 and by RT-PCR using CK20 specific primers.RESULTS Nine of 16 surgical specimens grew in the nude mice subcutaneously and/or orthotopically (4 of6 colorectal and 5 of 10 pancreatic cancer). Tumor cell content of the specimens and freezing of tissuespecimens are important factors influencing the growth of transplanted tumor. In the group of fresh tumortissues with greater than 50% tumor cell content, transplantation rate was 100% (3 cases of pancreatic cancerand 3 cases of colorectal cancer). The orthotopically transplanted tumors resembled the original tumormorphologically and biologically, including TAA expression such as CEA by immunohistochemistry, andCEA level in the serum of mice. Ki-67 labeling index and the expression of TAA especially K-ras, 17-1A and RA-96, were associated with the potential of tumor growth in nude mice. Micrometastases in the lungs andlivers of tumor bearing mice could be detected by immunostaining with biotinylated mab KL-1 and CK20-sepcific RT-PCR.CONCLUSION An orthotopic transplantation model for human colon and pancreatic cancer in nude micehas been established. The sensitive detection methods with CK-immunohistochemistry and CK20-RT-PCRwere also established to study xenotransplanted human cancer and its metastatic cancer cells in the liver andlung of nude mice. This study may be helpful in understanding the mechanism of cancer metastasis and indeveloping new diagnostic methods and therapeutic strategies for metastases.Acknowledgement The authors thank Dr. J. Luettges, Department of Pathology; Kiel University, for investigating thepathological characterics of the specimens; Dr. N. Zawazawa, Institute of Immunology, Kiel University, for the quantitativemeasurement of serum of CEA.