An experimental study on the adsorption of F(ab')2 molecules onto positively and negatively charged polystyrene beads is described. Adsorption isotherms at low ionic strength and different pH were performed. In all cases the adsorption isotherms showed well-defined plateau. The positively charged polystyrene beads showed a higher adsorption of F(ab')2 molecules over a range of pH. This latex sample reached a maximum adsorption at pH 7, whereas the negatively charged sample showed a maximum at pH 5. The isoelectric points of the F(ab')2-PS complexes were around 7.5 and 4.5 respectively. The differences in the colloidal stability of the sensitized beads are not explained by the electrophoretic mobility values. The colloidal stability of the F(ab')2-PS complexes is not only determined by the electrical charge of the complexes but also by certain properties of the protein molecules, which can be closely related to the conformational changes undergone by the F(ab')2 molecules in the adsorption process on polymer surfaces.