The authors investigated the influence of a preoperative irradiation on the rate of successful implantations and the growth behavior of human hypernephromas implanted into nude mice. No successful implantation was observed in hypernephromas preirradiated with a dose of 2000 rd, which was also confirmed by histologic examinations. The rate of successful implantation of 17 non-preirradiated hypernephromas (10/17) is below that of the total group, and the rate of successful implantation in serial passage (6/17 = 35%) is markedly inferior. The clinical observation that the incidence of local recurrences is reduced by a preoperative irradiation is explained by our experimental findings.