Thermal radiosensitization was tested in a pair of mouse cells (MB+ wild-type and MB-, DNA polymerase beta knockout cells) and in human breast carcinoma cells (MCF7 wild-type and C716 transfected to give elevated DNA polymerase beta expression). Results showed that neither reducing DNA polymerase beta (involved in base excision repair) nor increasing it had any significant effect on thermal radiosensitization. The data indicated that polymerase beta was not involved in thermal radiosensitization, and since hyperthermia is known as a radiation damage repair inhibitor, other repair pathways might be involved and need to be explored.