  The fundamental magnetic interactions of Pr$_{2}$Fe$_{17}$ are studied by
inelastic neutron scattering and anisotropy field measurements. Data analysis
confirms the presence of three magnetically inequivalent sites, and reveals an
exceptionally large value of the exchange field. The unexpected importance of
$J$-mixing effects in the description of the ground-state properties of
Pr$_{2}$Fe$_{17}$ is evidenced, and possible applications of related compounds
are envisaged.
