Interparticle interaction of monodisperse Fe3O4 nanoparticles has been experimentally investigated by dispersing the nanoparticles in solvents. With increasing the interparticle distances to larger than 100 nm in a controlled manner, the authors found that the blocking temperature (T-B) of the nanoparticles drops continuously and eventually gets saturated with a total drop in T-B of 7-17 K observed for 3, 5, and 7 nm samples, compared with their respective nanopowder samples. By carefully studying the dependence of T-B on the interparticle distance, the authors could demonstrate that the experimental dependence of T-B follows the theoretical curve of the dipole-dipole interaction. (C) 2007 American Institute of Physics.