A magneto-transformation method was developed for transferring plasmid DNA into Escherichia coli. Superparamagnetic nanoparticles (Fe3O4) having the saturation magnetization up to 68 emu/g were prepared by the method of co-precipitation at alkaline and thermal conditions, coated with polyethyleneimine (PEI), and then complexed with negatively charged DNA molecules. Plasmid DNA (pGEX-1 lambda T) attached to the PEI-coated magnetic nanoparticles could be transformed into E. coli DH1 by the application of a high magnetic field in pulses. Experimental results suggested that magnetic nanoparticles could well assist DNA delivery into bacterial cells. Factors that could significantly influence the transformation efficiency included the multiple pulsing in magnetic field and concentrations of DNA and magnetic nanoparticles. The highest transformation efficiency could be achieved by pulsing for three times in the 2.15 T magnetic field. The survival declined drastically at the additional pulses. Magneto-transformation of cells with 25 mu g DNA and 16 mu g magnetic nanoparticles resulted in transformants number in, the range from 1600 to 2480, corresponding to efficiency from 64 to 99 cfu/mu g DNA. Although the yield of transformants increased with input DNA, the transformation efficiency decreased with DNA dosage. Superparamagnetic nanoparticles played a crucial role in the bacterial transformation in strong magnetic field. Increasing the amount of input magnetic nanoparticles decreased both the survival and transformation efficiency. The higher dosage of nanoparticles led to more dead cells. (c) 2006 Elsevier Inc. All rights reserved.