There are some reports about combined effects of electromagnetic fields with known carcinogens, toxic physical or chemical agents. In previous study, we observed that magnetic fields (60 Hz, 50 mT) enhance cytotoxicity of mitomycin C on E.coli bacterium. However, it has not been clear about the effective condition of magnetic fields (magnetic flux density, frequency, exposure time etc.) on the potency of chemical agents. In this study, we investigated the effective density and the action mechanisms of magnetic fields on the enhancement of drugs potency. The result suggested that the enhancing effect of mitomycin C potency by exposure to magnetic fields depended on the magnetic flux density. The potency of cisplatin, widely used in clinical cancer chemotherapy, was also enhanced by exposure to 60 Hz, 50 mT magnetic fields. The analysis of the potency of supernatant remaining drugs in the culture medium indicated that the intracellular drug potency was increased and extracellular drug potency was decreased by exposure to magnetic fields. The values of drug potency revealed a significant inverse correlation between intracellular and extracellular cells. These results suggested that magnetic fields (60 Hz, 50 mT) change the permeability of cell membrane and influence the drug intake.