In this study, a method involving polyamidoamine dendron-modified magnetic nanoparticles (PAMAM-MNPs) along with application of an alternating magnetic field (AMF) was developed for inactivation of bacteria in water samples. The PAMAM-MNPs efficiently bound to Escherichia coli cells, resulting in magnetic recovery of cells from aqueous solutions. By applying the AMF (5 kW, 250 kHz) to the cell suspension, E. coli cells were successfully inactivated within 10 min in the presence of the MNPs, while no effect was observed in the absence of the MNPs. The use of PAMAM-MNPs could increase the inactivation rate of E. coli under the applied AMF. E. coli cells with PAMAM-MNPs stained by propidium iodide (PI) exhibited apparent fluorescence after exposure to the AMF, suggesting the occurrence of membrane damage in the cells because of direct heat transfer from the PAMAM-MNPs. Our technique can be used to address bacterial contamination with wide varieties of microorganisms in water samples. (C) The Electrochemical Society of Japan, All rights reserved.