A series of monodisperse sulfate polystyrene latex dispersions were covered with dioctadecyldimethylammonium bromide bilayers from small vesicles for particle sizes ranging from 76 to 412 nm. The zeta-potential of these bilayer-covered particles in water remains constant over the entire range of sizes tested. The kinetics of NaCl-induced flocculation for the bilayer-covered microspheres were obtained and the results used to construct curves of the logarithm of the stability ratio against the logarithm of electrolyte concentration. At a given salt concentration, stability increases, reaches a maximum, and then decreases as a function of size, Slopes of stability curves were calculated theoretically and compared with those obtained experimentally, The DLVO approximation by Reerink and Overbeek for an ideal colloid predicted an increase of slope with particle size which is not observed experimentally. This relationship can definitely be ruled out, In contrast, DLVO models which include aggregation at the secondary minimum qualitatively agree with the experimental dependence of colloidal stability on particle size.