  The discovery potential for detecting new heavy Majorana and Dirac neutrinos
at some recently proposed high energy $e^+e^-$ colliders is discussed. These
new particles are suggested by grand unified theories and superstring-inspired
models. For these models the production of a single heavy neutrino is shown to
be more relevant than pair production when comparing cross sections and
neutrino mass ranges.
  The process $ e^+e^- \longrightarrow {\nu} e^{\pm} W^{\mp}$ is calculated
including on-shell and off-shell heavy neutrino effects.
  We present a detailed study of cross sections and distributions that shows a
clear separation between the signal and standard model contributions, even
after including hadronization effects.
