[en] We review CP-violating effects in Z → 3 jet and Z → 4 jet decays, assuming the presence of CP-violating effective ZbbG and ZbbGG 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. (orig.)