To evaluate the influence of 5-thio-D-glucose on irradiation effect after hyperthermia, an experimental study was carried out using a total of 72 mice. A single dose of 3000 rads was delivered on skin of mouse tail after hyperthermia with or without intraperitoneal injection of 5-thio-D-glucose 2 hours before irradiation. The hyperthermia was carried out at 43$^{circ}C$ for 40 minutes just before the irradiation. The skin changes of the irradiated mouse tail were obseved from 7th to 31st post-irradiation days, and the scores were analyzed. The results are as follows: 1. Radiation damage on mouse skin increased significantly when radiation was combined with hyperthermia. 2. In the group of 5-thio-D-glucose and irradiation after hyperthermia, radiation damage on mouse skin significantly decreased than in the group without 5-thio-D-glucose. 3. 5-thio-D-glucose appears to be a potential radioprotective agent in clinical radiotherapy with hyperthermia.