Extremely low frequency magnetic field (ELFMF) produced by power lines and household electric appliances has been associated with increased incidence of cancers, as was suggested by several epidemiological studies,[1],. To test the genotoxic effects of ELFMF, the induction of micronuclei by exposure to ELFMF and/or X-rays was investigated by cytokinesis-block method in cultured Chinese Hamster Ovary (CHO) cells. Approximately 4×10,5, cells were plated in 10cm dishes, following exposure to an ELF magnetic field (60Hz, 5mT) for 24h. The cells were irradiated to 1Gy by X-rays. After the irradiation, cytochalasin B was added to the medium at a final concentration of 3 μg / mL. The cells were then exposed to an ELF magnetic field or placed in a normal incubator for 18h, which is 1.5times the length of their cell cycle. The micronuclei derived from acentric fragments or from whole chromosomes were evaluated with the help of immunofluorescent staining using antikinetochore antibodies from the serum of scleroderma (CREST syndrome) patients,[2,3],.