Magnetite (Fe3O4) nanoparticles, 8 to 9 nm in size, have been synthesized using an aqueous precipitation technique. X-ray diffraction and chemical titration confirm a single cubic spinel phase with expected stoichiometry. Superparamagnetic behavior has been observed in pressed pellets of the nanoparticles above 200 K. Spin-dependent tunneling through adjacent particles has led to a negative magnetoresistance, -8.6% at 200 K and -4.5% at 300 K in a 70 kOe field. This is caused by the field-induced alignment of the nanoparticle magnetization directions. (C) 2003 American Institute of Physics. [References: 15] 15