Carbon nanotubes are excellent electron emitters due to their sharp geometry and high electrical conductivity and mechanical stability. It has previously been shown that an electron beam hitting the tip of a nanotube biased near the threshold of field-emission can stimulate the emission of a large number of electrons from the nanotube tip. Here we present a detailed characterization of this so-called electron-stimulated field-emission phenomenon. Electron gains of up to 2300 were obtained. We also discuss possible direct and indirect electron-nanotube interaction mechanisms responsible for this high gain. This effect has a good potential for vacuum nanoelectronic applications.