Human gastrointestinal cancer xenografts were established in the nude mouse. Grafts were accomplished with gastric adenocarcinomas, gastric leiomyosarcoma, histiocytic lymphoma of the stomach and gallbladder, pancreatic tumors, colonic cancers and cell lines of duodenal (HUTU-80) and pancreatic (HS-766-T) cancers, melanoma (SK-Mel-5), and murine metastasizing Lewis lung carcinoma. The rate of successful xenografting of these tumors varied from virtually 100% with colon and duodenal cancer, 50% for a pancreatic cancer (P-1), to only 17% for gastric adenocarcinoma. Pancreas and colon adenocarcinomas have been maintained by successive xenotransplantation over 16 and 19 months, respectively. Human xenografts retained morphological identity with tissues of origin through several transplant generations and shared some of their ultrastructural characteristics but did not metastasize. Rodent xenografts, of heterogenous origin were characterized by differences in the duration of the latent period and in the rate of their initial development as described by the average doubling times and average slopes (B) of their growth curves. Differences between B of the Lewis lung carcinoma and all of the human xenografts and between B of a pancreatic adenocarcinoma and three other neoplasms were significant (P less than 0.05 to 0.04). Labeling indices determined for 14 cancer transplants were in the range of previously reported data for similar neoplasms in patients or other xenograft systems. These findings suggest that the nude mouse model can be used to evaluate endogenous properties of gastrointestinal cancers and their responses to exogenous agents.