The surface force apparatus was used to study the effects of polyelectrolyte concentration on the interaction between mica sheets immersed in aqueous polyelectrolyte solution. We have found that at low polyelectrolyte concentration, adsorption of cationic polyelectrolyte onto negatively charged mica leads to extremely flat adsorbed layers in solution of low ionic strength. There is a repulsion between the surfaces at separations between about 100 and 10 nm. This force is of electrostatic origin when the polyelectrolyte concentration is lower than that needed to neutralize the surface charge. When increasing the polyelectrolyte concentration above the charge neutralization point, cnp, only a slight additional adsorption takes place in low-ionic-strength solution. In these systems, a steric component in the interaction force appears. The adsorption at the cnp is approximately 20% below that at the plateau level in the adsorption isotherm, which is found by ESCA measurements.