The iron storage protein ferritin consists of a spherical polypeptide shell (apoferritin) surrounding a 6-nanometer inorganic core of the hydrated iron oxide ferrihydrite (5Fe2O3.9H2O). Previous studies have shown that the in vitro reconstitution of apoferritin yields mineral cores essentially identical to those of the native proteins. A magnetic mineral was synthesized within the nanodimensional cavity of horse spleen ferritin by the use of controlled reconstitution conditions. Transmission electron microscopy and electron diffraction analysis indicate that the entrapped mineral particles are discrete 6-nanometer spherical single crystals of the ferrimagnetic iron oxide magnetite (Fe3O4). The resulting magnetic protein, "magnetoferritin," could have uses in biomedical imaging, cell labeling, and separation procedures.