The structural organization of paramagnetic centers in biomolecules can be predicted on the basis of a quantitative study of their magnetic interactions. These studies are usually carried out within the framework of the so-called point dipole approximation, in which the delocalization of the magnetic moments over the centers is ignored. In this paper, we examine how this delocalization can be taken into account in the spin Hamiltonian describing the magnetic interactions between polynuclear paramagnetic centers. A local spin model is described and applied first to a system made of a dinuclear center interacting with a mononuclear center and then to a system comprising two dinuclear centers. In both cases, the EPR spectrum calculated from the local spin model is compared to that given by the point dipole model for different geometrical configurations. The model is illustrated by a detailed quantitative study of the magnetic interactions between the molybdenum center and one [2Fe-2S] center (center 1) in the enzyme xanthine oxidase. These studies emphasize the effective character of some important structural parameters given by numerical simulations of EPR spectra based on the point dipole approximation.