The magnetoresistance of (Au_1-xCu_x) Mn spin glass alloys has been studied for large variations of copper and manganese atom concentrations at temperatures above the freezing temperature. The magnetoresistance has been observed to depend strongly on the concentration of both the magnetic (Mn) and non-magnetic (Cu) impurities. The variation in the Cu concentration for a constant value of the Mn concentration has enabled the authors to study the dependence of magnetoresistance on the strength of the RKKY interaction.