  Determination of the top-quark Yukawa couplings in the process e+ e- -> t
\bar{t} Z has been studied for the high luminosity option (\int L=500\fbinver)
of a linear e+e- collider. Polarization of the electron beam has been
considered. The method of optimal observables has been adopted to determine the
couplings and to disentangle different models. It has been found that it could
be possible to discriminate between the Standard Model scalar Higgs boson, pure
pseudoscalar and the Higgs boson of mixed CP at about 12 \sigma level for
\sqrt{s}=1.TeV for the Higgs boson close to the production threshold both for
the leptonic and hadronic top-quark decay channels. However, \sqrt{s}=.5 TeV
collider turned out to have too small energy for tests of the top-quark Yukawa
coupling because of its limited production rate (at most 2.5 \sigma effect).
Possible tests of CP violation in the scalar sector have been considered.
