  We discuss the potential of NLC500 and NLC1000 to probe Z-Z' mixing and mass
by the reaction e^+e^- -> W^+W^- with longitudinally polarized electrons. We
perform a model-independent analysis of the deviations from the Standard Model,
and apply it to a specific class of extended weak gauge models. Results
indicate that the corresponding bounds at the NLC500 complement the present
ones obtained from LEP1, and rapidly become quite stringent at the higher
energy of the NLC1000. Also, we emphasize the importance of initial beam
polarization in improving the sensitivity to mixing.
