It turned out from room temperature electrical resistivity and magnetoresistance studies of electrodeposited Co metal and Co(Ru) dilute alloys that the incorporation of a small amount of Ru into Co significantly reduces the anisotropic magnetoresistance (AMR). The influence of Ru on the AMR of Co is explained by a drastic change of the asymmetry of d-band spin-up and spin-down electronic states at the Fermi level as revealed also by recent electronic band-structure calculations on Co-rich Co-Ru alloys.Introduction. -Extensive magnetoresistance (MR) studies have already been performed for magnetic/non-magnetic Co/Ru multilayers [1-4] and sandwiches [3,[5][6][7]. Although a clear giant magnetoresistance (GMR) effect has been observed in each case, its magnitude remained rather small (typically 0.1% at room temperature). This is in contrast to theoretical predictions made on the basis of the resistivity change caused by Ru impurities in a Co matrix [8]. It has been indicated in a recent study [5] that for Co/Ru multilayers and sandwiches, either sputtered or evaporated, a strong chemical intermixing occurs at the Co/Ru interface. It was argued that this intermixing, promoted by the complete mutual solubility of Co and Ru in the hexagonal close-packed (hcp) structure [9], leads to a strong reduction of both the GMR and the interlayer exchange coupling. In fact, the latter was found to be more than an order of magnitude smaller [5] than the theoretical prediction [10]. In view of the fact that a fairly large GMR is expected for the Co-Ru system, on the basis of the large spin asymmetry at the Fermi level [8], the observed GMR is, indeed, also very small.However, whereas the interlayer exchange coupling can certainly be very effectively reduced by interfacial intermixing [11][12][13], a weak antiferromagnetic coupling or the lack of coupling does not necessarily results in very small GMR [14]. For example, it has been demonstrated