Interactions between two negatively charged mica surfaces across aqueous solutions containing various amounts of a 10% charged cationic polyelectrolyte have been studied. It is found that the mica surface charge is neutralised when the polyelectrolyte is adsorbed from a 10-50 ppm aqueous solution. Consequently no electrostatic double-layer force is observed. Instead an attractive force acts between the surfaces in the distance regime 250-100 Å. We suggest that this attraction is caused by bridging. Additional adsorption takes place when the polyelectrolyte concentration is increased to 100 and 300 ppm, and a long-range repulsion develops. This repulsive force is both of electrostatic and steric origin. The polyelectrolyte layer adsorbed from a 50 ppm solution does not desorb when the polyelectrolyte solution is replaced with an aqueous polyelectrolyte-free solution. Injection of sodium dodecyl sulphate, SDS, into the measuring chamber to a concentration of about 0.01 cmc (8.3¥10-5 M) does not affect the adsorbed layers nor the interaction forces. However, when the SDS concentration is increased to 0.02 cmc (0.166 mM) the adsorbed layer expands dramatically due to adsorption of SDS to the polyelectrolyte chains. The sudden swelling suggests a cooperative adsorption of SDS to the preadsorbed polyelectrolyte layer, and that the critical aggregation concentration between the polyelectrolyte and SDS at the surface is about 0.02 cmc. The flocculation behaviour of the polyelectrolyte in solution upon addition of SDS was also examined. It was found that 0.16-0.32 mol SDS/mol charged segments on the polyelectrolyte is enough to make the solution slightly turbid.