The poor 5-year survival rate of patients with malignant colorectal tumors calls for clarification of the question as to whether it is advisable to carry out postoperative adjuvant chemotherapy. In the present study a simple "in vivo" test is described with which the sensitivity of individual human adenocarcinomas of the colon, primary xenotransplanted onto syngeneic "nude"-mice suffering from congenital thymic dysplasia, to cytostatic therapy with 5-FU and VCR could be determined. Five of six primary xenotransplanted tumors showed a significant reduction in size following combined therapy with these drugs in doses comparable to those administered clinically, whereas one of the transplanted tumors remained unresponsive to therapy. On the basis of these results it is absolutely necessary that a comparison be carried out between the rate of tumor remission in groups of patients and animals undergoing the same chemotherapy.