  We review CP-violating effects in Z --> 3 jet and Z --> 4 jet decays,
assuming the presence of CP-violating effective Z b bbar G and Z b bbar G G
couplings. Longitudinal beam polarization is included in the studies. We
propose a direct search for such CP-violating couplings by using various CP-odd
observables. The data of a future linear collider running at the Z-resonance in
the so-called GigaZ option should give significant information on the
couplings. Finally we show that stringent bounds on the mass of excited b
quarks can be derived if appropriate couplings are of a size characteristic of
a strong interaction.
