The dependence of giant magnetoresistance on the composition of ferromagnetic face‐centered cubic precipitates in an Ag matrix grown epitaxially on MgO {100} is described. These compositions were chosen from the alloy ranges Ni0.6Cu0.4→Ni→NixCo1−x→ Co→Fe0.2Co0.8to give a number of valence electrons per atom,n=n3d+n4s, in the ferromagnetic precipitates that ranged from 8.8 to 10.4. This corresponds to a decrease of the saturation magnetization from ∼1500 emu/cm3to approximately zero. To reduce the influence of other variables the concentration of the precipitates was kept constant (at 23.5 vol %) and their size approximately so. Corrections are applied to adjust for some remaining variation in precipitate size. With other factors held constant, there is a steady increase in both the magnetoresistance ratio and the magnetic‐field‐dependent resistivity with increase in the saturation magnetization of the alloys, i.e., with increase in the spontaneous magnetization of the single‐domain precipitates. ©1995 American Institute of Physics.