Agglomerated cobalt magnetic nanoparticles coated with carbon, dispersed in a paraffin matrix, were prepared and investigated by FMR (ferromagnetic resonance) at room temperature. Four samples with different C/Co content, ranging from 0.175 to 1.011, dispersed at low concentration in paraffin were investigated. Very intense and broad FMR spectra with different intensities, line widths and positions of the resonance fields were recorded for the samples. A strong dependence of the FMR signal intensity and resonance on the concentration of magnetic nanoparticles was observed. Various magnetic interactions affecting the observed FMR spectra have been analyzed. It was found that with increasing concentrations of magnetic nanoparticles the magnetic dipole interaction between the agglomerates plays a more important role.