The training effect in polycrystalline exchange bias Co/CoO bilayers has been investigated with low-temperature magnetic force microscopy (MFM). After field cooling the bilayer to 13 K, no MFM contrast was detected but a clear MFM contrast related to the domain structure appears after the first magnetization reversal of the ferromagnetic Co layer. Once formed, the ferromagnetic domains survive even at very large fields and cannot be erased by the application of a magnetic field along the cooling field direction. On the other hand, it is possible to partially remove the magnetic domains by performing a hysteresis loop with a magnetic field perpendicular to the cooling field direction. (C) 2008 American Institute of Physics.