Is there a chance to find heavy neutrinos in future lepton colliders?

We examine two processes, the neutrino production process e+e− → Nν and the inverse neutrinoless double-β decay process e−e− → W−W− as possible places for discovering heavy neutrinos in future lepton linear colliders. The heavy neutrino parameters are bound from existing experimental data. We use only one important theoretical input, the lack of a Higgs triplet. As a consequence the neutrinos must have different CP parities. In such models the existing experimental bounds for mixing parameters still give a chance that heavy neutrinos can be observed in future e+e− and e−e− colliders.