A novel, efficient DNA delivery and transfection agent has been developed by chemically attaching short chain polyethyleneimine (PEI) molecules ( M. W. = 423 and 1800) to the surface of composite iron oxide nanoparticles ( 250 nm in diameter). These particles are not only capable of delivering DNA but also can be driven by an external magnetic field. During DNA binding experiments, a remarkably low N : P ratio of < 1 was recorded. This is due to the more effective distribution of the PEI molecules across the surface of the nanoparticles in comparison to particles where PEI is attached to the surface via simple electrostatic interactions. This new attachment strategy allows the use of smaller amounts of PEI with potential reduction in toxic side-effects.