Background: Radioiodine(131I), a major component of nuclear fallout and a valuable therapeutic agent for thyrotoxicosis and thyroid cancer, has been regarded as a mutagen or a carcinogen without any convincing evidence. To evaluate the genotoxicity of radioiodine(131I) we performed a micronuclei test mice bone marrow. Materials and methods: Mice (ICR strain, 25∼30 g) were divided to 4 groups: control, group 1 (0.17 mCi/kg, usual therapeutic dose for thyrotoxicosis), group 2 (1.67 mCi/kg, usual therapeutic dose for thyroid cancer), and group 3 (16.67 mCi/kg, usual accumulated dose causing bone marrow suppression). 131I was administered intraperitoneally. Ten mice of each group were sacrificed at days 1 and 3. Bone marrow were smeared and stained with May-Grunwald Giemsa method. One thousand polychromatic erythrocytes (PCE) and normochromatic erythrocytes (NCE) were counted under the light microscope, and the number of micronucleated PCEs were recorded. Results: The frequency of micronuclei in PCE (and NCE in parenthesis) in the control group was 0.25±0.07 (0.23±0.07)% in day 1 and 0.24±0.07 (0.21±0.07)% in day 3. Those in group 1 was 0.27±0.1 (0.23±0.09)% in day 1 and. 0.28±0.07 (0.25±0.06)% in day 3. Micronuclei was noted in 0.29±0.08 (0.26±0.09)% in day 1 and 0.31±0.05 (0.29±0.06)% in day 3 in group 2, and in 0.32±0.06 (0.25±0.09)% in day 1 and 0.33±0.08 (0.3±0.06)% in day 3 in group 3. There was no difference in the frequency of micronuclei between each groups (p〉0.05) Conclusion: Radioiodine (131I) did not cause any genotoxicity in mice bone marrow even at the large dose (16.67 mCi/kg).