Interactions between negatively charged latex particles in the presence of cationic linear poly(ethylene imine) (LPEI) were studied with atomic force microscopy (AFM) and electrophoresis. Forces were measured directly with the recently developed multiparticle colloidal probe technique, which permits colloidal particles to attach to the cantilever in aqueous dispersions in situ and ensures a large surface area during experiment. It was observed that the forces vary from repulsive to attractive and back to repulsive with increasing polymer dose. The repulsive forces are due to overlap of the diffuse layers around charged surfaces. The attractive forces are independent of the ionic strength and the molecular mass of the polymer and can be rationalized in terms of classical van der Waals interactions. Additional electrostatic attractive forces due to patch-charge heterogeneities observed in other particle–polyelectrolyte systems are absent here. Their absence indicates that the adsorbed layer of LPEI has a high lateral homogeneity.