Mutagenic effects of static and time-varying (50 Hz) magnetic fields (MFs) were investigated using bacteria, yeast and fruit fly. Exposure to a 5-Tesla static MF caused an increase in the gene conversion frequency in yeast and an increase in the somatic recombination in fruit fly while the frequency of point mutation was not affected in yeast, fruit fly or bacteria. When exposed to a 50 Hz, 40 mT MF, the point mutation frequency in bacteria was not affected. Point mutation and gene conversion frequency in yeast was also not affected by exposure to a 50 Hz MF of 30 mT. On the other hand, somatic recombination was significantly increased by exposure to a 20 mT field in fruit fly. These results show that the mechanisms of mutagenicity of static and 50 Hz field are different. Whenever mutagenic effects which can cause cancer were detected by our test systems, the degree of the effects is very small though we use field strength which is 1,000-10,000 times greater than the ones to which we are actually exposed to. This suggests that the possible carcinogenic effect of MFs are practically negligible for the majority of human beings.