Fibre and filler flocculation, filler retention and drainage, induced by several polymeric retention aids, were compared in laboratory experiments on mixtures of kraft fibres and two calcium carbonate fillers. Some experiments were also per-formed on thermomechanical pulp and de-inked pulp fibres. Flocculation was measured by a focused beam reflectance measurement probe. It was found that flocs induced by polyethyleneoxide (PEO) and cofactor broke up with time and shear and could not be reformed subsequently. Floc strength was the highest for PEO and the weakest for polyethylenimine and polyaluminium chloride. When comparing filler retention under optimal flocculation conditions, we found similar filler retention for all retention aids. Salt did not affect drainage for cationic retention aids, but reduced the drainage rate for PEO. Drainage with PEO was considerably slower than for other retention aids.