Low efficiencies of nonviral gene vectors, the receptor-dependent host tropism of adenoviral or low titers of retroviral vectors limit their utility in gene therapy. To overcome these deficiencies, we associated gene vectors with superparamagnetic nanoparticles and targeted gene delivery by application of a magnetic field. This potentiated the efficacy of any vector up to several hundred-fold, allowed reduction of the duration of gene delivery to minutes, extended the host tropism of adenoviral vectors to nonpermissive cells and compensated for low retroviral titer. More importantly, the high transduction efficiency observed in vitro was reproduced in vivo with magnetic field-guided local transfection in the gastrointestinal tract and in blood vessels. Magnetofection provides a novel tool for high throughput gene screening in vitro and can help to overcome fundamental limitations to gene therapy in vivo.