Ionizing radiation is one of the most extensively studied carcinogens. In contrast to the detrimental effects of high-dose radiation in carcinogenesis, the biological effects of low-dose radiation remains poorly understood. In this study, we introduced adult wts/ + heterozygotes of Drosophila melanogaster as transgenic model organisms to determine the tumorigenic activity of low-dose radiation. The warts (wts) gene is a tumor suppressor gene in mice and humans that is directly involved in cell cycle regulation. Fruit flies at the first larval stage were subjected to ionizing radiation, and then tumorigenic activity was evaluated as the frequency of observed warts tumorous mosaic clones in adult flies. Low-dose irradiation alone did not cause tumorigenesis in our system. In combined treatment with a chemical carcinogen, chronic irradiation at 0.2 Gy decreased the frequency of tumorous clones induced by cisplatin. These results suggest that low-dose radiation alone is not deleterious but beneficial in tumorigenesis induced by a chemical carcinogen.