  We examine the e-e- -> W-W- lepton number violating process in the frame of
the Standard Model with additional right-handed neutrino singlets. We give
results in the framework of the `see-saw' as well as other models, where there
is no relation between neutrino masses and the mixing matrix elements. The
cross section for the `see-saw' models is negligible because they predict in a
natural way very small electron-heavy neutrino mixing angles. However, there
exist other models, in which the electron-heavy neutrino mixing angles are free
parameters and can be large. Taking into account the present experimental
bounds on mixing angles the large cross section for the e-e- -> W-W- is still
acceptable.
