Cobalt-silver (Co-Ag) core-shell nanoparticles with different silver thicknesses were prepared by the microemulsion method in a two-step reduction process. Transmission electron microscopy (TEM) characterization revealed the almost monodispersity and nanometric size (in the range 3-5 nm depending on the shell thickness) of the synthesized nanoparticles. However, it was the use of high-resolution TEM that revealed the correct core-shell formation of the nanometric material. The selected area electron diffraction pattern indicated the fcc (face-centered cubic) and hcp (hexagonal close packed) nature for silver and cobalt, respectively. Cyclic voltammetry also allowed the correct core-shell formation to be assured. The magnetic properties revealed the presence of both superparamagnetic and ferromagnetic contributions. Because of the lack of methodology, it was necessary to develop a method to measure the magnetotransport properties of the prepared nanoparticles. The strategy which followed was successful as it was possible to measure these properties: giant magnetoresistance values of 0.1% at room temperature were obtained. The numerical analysis of magnetic and magnetoresistance data indicated the presence of superparamagnetic particles showing interaction among the magnetic moments.