We develop in this paper two methods, based on different physical concepts, to quantify the uptake of magnetic nanoparticles in biological cells. The first one, magnetophoresis, is based on the measurement of the velocity of magnetically labeled cells submitted to a magnetic field gradient. The second one quantitates the particles' electronic spin using an electron paramagnetic resonance experiment. We show a quantitative agreement between both methods for macrophagic cells. The uptake kinetics and uptake capacity are discussed for macrophagic cells and other cell lines.