In order to evaluate the influence of static magnetic fields (SMF) on the progression of cell cycle as a monitor of presumptive genotoxicity of these fields, the effects of a 15 mT SMF on cell cycle progression in rat bone marrow stem cells (BMSC) were examined. The cells were divided into two groups. One group encountered SMF alone for 5h continuously but the other group exposed with X ray before treatment with SMF. The population of cells did not show any significant difference in the first group but the second group that was exposed with acute radiation before encountering SMF showed a significant increase in the number of cells in G(2)/M phase. So SMF has intensified the effects of X ray, where SMF alone, did not had any detectable influence on cell cycle. These findings suggest that magnetic fields (MF) play their role by increasing the effects of genotoxic agents and because of the greater concentration of free radicals in the presence of radical pair producers, this effect is better detectable.