Not only the effect of cytostatic drugs and hormones, but also that of ionizing radiation can be tested after transplantation of human tumoral tissue into nu/nu mice with thymic aplasia. In this study, the effects of high voltage irradiations dosed up to 60 Gy (fractionation 2 X 5 Gy per week) to three carcinomas of the endometrium, three carcinomas of the cervix, three ovarian carcinomas, and one mammary metastasis of an immunoblastic sarcoma were observed during a period of 120 days. This irradiation plan could be performed without problems in eight cases, in two cases it had to be stopped after 50 Gy. Nine tumors decreased significantly in size, reaching sizes which were below those measured before irradiation. In six among the nine cases, the tumors grew again after different periods of observation. In case of rather long periods of observation, those points within the curves of development can be taken as efficacy indices which are representing the lowest or the initial values of tumor sizes. In another study, the effects of high voltage irradiation to tumors of the nude mouse are correlated to those found in the corresponding patients.