  We study the single production of heavy neutrinos via the processes $e^-e^+
\to \nu N$ and $e^-\gamma \to W^- N$ at future linear colliders. As a base of
our considerations we take a wide class of models, both with vanishing and
non-vanishing left-handed Majorana neutrino mass matrix $m_L$. We perform a
model independent analyses of the existing experimental data and find
connections between the characteristic of heavy neutrinos (masses, mixings, CP
eigenvalues) and the $m_L$ parameters. We show that with the present
experimental constraints heavy neutrino masses almost up to the collision
energy can be tested in the future experiments.
