This report describes the measurements on individual nitrogen-doped multi-walled carbon nanotubes mounted on tungsten support tips. Both the current-voltage characteristics and the energy spectrum of the emitted electron beam revealed field emission behavior similar to that shown for the best metallic emitters. The experimental results correspond with numerical calculations based on electron microscopy images of the nanotubes. The work function was derived and amounted to 5.0 eV. Finally, it is shown that the emission process is highly sensitive to small amounts of impurities deposited on the surface of the nanotube. (C) 2004 Elsevier B.V. All rights reserved.