Characteristics of globulin-immobilized styrene/acrylamide/acrylic acid copolymer [poly(St/AAm/AA)] fine particles and their flocculation by an antigen-antibody reaction were investigated as a function of acrylamide (AAm) content. Immobilization of globulin was carried out through octamethylenediamine with a carbodiimide reaction. The increase in AAm content in particles decreases the amounts of bovine gamma-globulin (B gamma G) immobilized, shifts the isoelectric points of B gamma G-immobilized poly(St/AAm/AA) particles to acidic pH, and increases the dispersion stability of B gamma G-immobilized poly(St/AAm/AA) particles. The flocculation rates of antihuman epidermal growth factor (hEGF) immunoglobulin (IgG)-immobilized poly(St/AAm/AA) fine particles of low AAm content by the antigen-antibody reaction are higher than that of anti-hEGF IgG-adsorbed polystyrene particles, whereas those of high AAm content are lower and decreased with increasing AAm content.