ESR investigations on milled multiwalled carbon nanotubes are reported. The ESR spectra of pristine and milled nanotubes consists of two resonances, a wide line located at g>3.0 assigned to magnetic particles (catalysts residues) and a broad line with a peak to peak linewidth of about 10−2 T located at g=2.05 and assigned to the interaction between the conducting electrons delocalized over carbon nanotubes and magnetic ions. It was observed that this line depends on the milling time. For short milling times (up to 125 minutes) both the resonance linewidth and the g-factor increase as the milling time is increased. Longer milling times resulted in a decrease of both the g-factor and resonance linewidth as the milling time was increased. This behavior was assigned to the removal of the magnetic nanoparticles from nanotubes. No resonance line due to the destruction of carbon nanotubes was observed.