The influence of extremely low frequency (ELF) magnetic fields has been studied using the Ames mutagenesis tests since mutation may be an early event in carcinogenesis. In the experiments, genetically engineered strains of Salmonella Typhimurium (TA102 and TA98) were used. The 60-Hz magnetic field distribution was made almost uniform within a range from 13 μT(rms) to 15 μT(rms) where the test samples were located. Moreover, the test samples were subjected to an ultraviolet light with a wavelength of 365 nm. The results on mutageneity were negative with 60-Hz magnetic fields. However, a significant amount of mutations were observed when TA102 was exposed to 365-nm ultraviolet radiation.