Using human renal carcinoma heterotransplanted in nude mice, the effects of the combined use of hyperthermia and interferon (natural human interferon-alpha; IFN-alpha, and recombinant interferon-gamma; IFN-gamma) were examined. The hyperthermic device used was the Clini-Therm, Mark VII, 915 MHz, microwave. The administration of interferon was started immediately before hyperthermia and carried out on consecutive days. The combined use of hyperthermia and IFN was compared with hyperthermia and with IFN alone. The combined use of hyperthermia and IFN-alpha was shown to have significant anti-tumour effects compared with the former or latter alone. Of 10 nude mice (10 implanted tumours) used for the experiment, 5 showed complete disappearance of the implanted tumours and the others showed prolongation of survival. Combined treatment with IFN-gamma and hyperthermia showed no significant effect when compared with other treatment groups. Combined treatment with local hyperthermia and IFN-alpha may have some application to the clinical management of renal carcinoma.