The competitive adsorption of two biocides, the cationic polysaccharide (chitosan, CH) and some cationic bilayer fragments (composed of dioctadecyldimethylammonium. bromide, DODAB), onto oppositely charged polystyrene sulfate (PSS) or poly(methyl methacrylate) (PMMA) particles is described. CH and/or DODAB effects on particle stabilization/flocculation are characterized from the following: (1) DODAB adsorption isotherms from bilayer dispersions onto PMMA particles in the presence or absence of CH or 10 mM NaCl; (2) effects of particle number density, DODAB or CH concentration on mean size, and zeta-potentials of PSS or PMMA particles; (3) visualization of positively stained cationic moieties on PMMA particles by transmission electron microscopy. Driving forces for DODAB bilayer adsorption were the electrostatic and the hydrophobic attraction between latex and bilayer, whereas, for CH, charge neutralization (overcompensation) led to particle flocculation (stabilization). Both cationic species seem to adsorb not only as patches but also as stiff and extended protrusions from the particle surface, e.g. at low particle number densities (Np) and relatively high chitosan or DODAB concentrations, DODAB bilayer fragments attaching on particles also from their hydrophobic edges. At a given Np and low (high) CH concentrations there was DODAB (CH) displacement by CH (DODAB). The entropic penalty paid by CH for adsorption on latex with charge overcompensation did not hold for the also stiff and flat DODAB bilayer fragments which displaced CH and attached via hydrophobic and/or electrostatic interaction to the latex surface. zeta-Potentials range for particles varied from 35 to 40 (DODAB only) up to 65-75 mV (CH only). Together, DODAB and CH had to be added to the particulates in minute doses for preservation and sterilization against bacteria contamination.