In this study, single-stranded DNA is employed as a model polymer to investigate the effect of polyelectrolyte coatings on colloid stability. By use of homologous series of negatively charged DNA homopolymers ranging in size from monomers to molecules 1400 nucleotides in length (MW = 330 to 460,000, respectively), it was found that there is a critical polymer length (CPL) required for the stabilization of polystyrene latex microspheres suspended in an aqueous solution at high salt concentration (0.35-1 M NaCl). The CPL corresponds to one to two persistence lengths, suggesting that relatively little molecular flexibility is required for the development of adsorbed layers capable of stabilizing particle suspensions. For polymers larger than the CPL, the stability of the particle suspension increases with increasing chain length at high surface coverage. The CPL depends weakly on the background NaCl concentration, implying that these molecules stabilize particles through a combination of steric and electrostatic-or electrosteric-repulsive forces. The results indicate that polymer flexibility is an important factor in the ability of polymer molecules to stabilize colloidal particles, particularly for systems where the time scale for coagulation is short relative to the time scale for polymer relaxation on the surface.