The magnitude andsignof the conduction electron polarization in rare earth intermetallic compounds hasbeen determined with the use of NMR techniques. Large, temperature-dependent Knight shifts of theAl27NMR were found inXA12(X ≡ rare-earth ion). These observations may be explained by assuming that anegativeexchange interaction of ∼0.1 ev exists between the localizedfelectrons of the rare-earth ion andthe conduction electrons.