Anti-BSA was separated from the antisera developed in rabbit with BSA-coupled polymeric microspheres. BSA was coupled by simple adsorption method, and the simple polystyrene particles modified with small amount of comonomers were used. The amount of separated antibodies was quantified with turbidity changes by particle agglutination. The sensitivity was maximal when the surface was half-covered with BSA and when the comonomer was strong acid.