We first examined two lines of transgenic mouse, gpt delta and Muta, for sensitivity of radiation-induced mutations in spleen. The gpt delta mouse could detect mutations induced by 2 to 8 Gy of gamma-rays with 2-times higher sensitivity than Muta mouse. The reason seemed to be that radiation induces predominantly deletion type mutation which is rather rare in natural background and gpt delta mouse can detect exclusively this deletion type mutation, whereas Muta mouse detects all types of mutation including the deletion. Next, we applied the gpt delta mouse to study mutation induction by tritiated water. A half ml of saline containing 0, 266 or 532 MBq of tritiated water was administered to the mice intraperitoneally and mutations in spleen were examined 7 days later. The estimated absorbed doses were 0, 3 and 6 Gy, respectively. With 3 Gy, the mutant frequency was elevated approximately 2-fold above control level, and it remained at a similar level with 6 Gy. These indicate that gpt delta mouse could be a good model animal to study genotoxicity of tritium.