Nanoparticles (NPs) of synthetic hydroxyapatite (Hap) and natural bone mineral (NBM) are rendered magnetic by treatment with iron ions using a wet-chemical process. The magnetic NPs (mNPs), which are about 300 nm in 1 diameter, display superparamagnetic properties in a superconducting quantum interference device, with a saturation magnetization of about 30 emu g(-1). X-ray diffraction and transmission electron microscopy reveal that the magnetic properties of the NPs are the result of the hetero-epitaxial 1 growth of magnetite on the Hap and NBM crystallites. The mNPs display a high binding affinity for plasmid DNA in contrast to magnetite NPs which do not bind the plasmid well. The mHap and mNBM NPs result in substantial increases in the transfection of rat marrow-derived mesenchymal stem cells with the gene for glial cell line-derived neurotrophic factor (GDNF), with magnetofection compared to transfection in the absence of a magnet. The : amount of GDNF recovered in the medium approaches therapeutic levels despite the small amount of plasmid delivered by the NPs.