The adsorption of single-stranded polyadenylic acid (poly dA) (30 bases) onto cationic polystyrene latex particles was studied. The latex particles were prepared by soap-free emulsion copolymerization of styrene and vinyl benzylamine hydrochloride. Adsorption experiments were carried out using a fluorescein-labeled polyadenylic acid and adsorption isotherms were established for constant ionic strength in a pH range of 4-9. Poly dA adsorption was optimal at low pH and decreased upon increasing pH. Desorption of oligodeoxyribonucleotides was studied by washing the latex particles and by changing the pH. The adsorption of polyadenylic acid onto cationic latexes is interpreted based upon the contributions of hydrophobic and electrostatic interactions.