Out of 83 human colorectal and 44 stomach cancers transplanted subcutaneously in nude mice, tumor take was observed in 78 and 68 %, respectively. Progressive tumor growth (product of two tumor diameters . > 60 mm2 after 90 days) was found in 49 and 32%. Serial passage was done in 46 colorectal and 17 stomach cancers. In 5 out of 6 esophageal cancers tumor take was found, and serial passage was performed in 4 tumors. Tumor stage was the most important factor for the take rate. Metastasized or locally advanced tumors of the large bowel and stomach were grown in 89 and 54%, respectively, which was significantly higher than in the localized stage. The take rate was independent of the degree of differentiation, the amount of fibrous tissue, CEA content, sex and tumor localization, except for tumors originating in the cardia in comparison to other parts of the stomach. The similarity of the xenografts in serial passage in comparison to the donor tumor was shown by histological and immunological (CEA) examinations. The presence of human isoenzymes such as esterase-D and LDH demonstrate the human origin of the tumors. However, most of the xenografts were growing more rapidly in the serial passage than in early passages. 31 comparisons of tumor response in the patient and in nude mice were made in 20 different tumors. The highly correct prediction rate for resistance (100%) and sensitivity (86%) validates the xenograft system. 34 colorectal, 14 stomach and 3 esophageal cancers were selected and characterized as tumor models. This tumor panel is continuously available for therapeutic and biological studies. For surgical purposes the xenografts can also be grown in athymic nude rats. The most relevant model for the clinic is available if the human tumors are implanted at the site of tumor origin.