Abstract: The invention relates to magnetic nanoparticles, their production, and their use. The object of the invention is to provide nanoparticles capable of specifically forming bonds to intracellular biomacromolecules even in the intracellular region of cells, so that separation is possible by exposure to an exterior magnetic field. This is accomplished by means of magnetic nanoparticles having biochemical activity, consisting of a magnetic core particle and an envelope layer fixed to the core particle, and including a compound of general formula M-S-L-Z (I), the linkage sites between S and L and L and Z having covalently bound functional groups, wherein M represents said magnetic core particle; S represents a biocompatible substrate fixed to M; L represents a linker group; and Z represents a group comprised of nucleic acids, peptides or proteins or derivatives thereof, which group has at least one structure capable of specifically binding to a binding domain of an intracellular biomacromolecule.