Soluble-insoluble forms of monoclonal antibody, which can be rendered either soluble or insoluble by simple adjusting the temperature or pH, have been prepared by covalently coupling antibody to poly(N-isopropylacrylamide) or to copolymer of methacrylic acid and acrylamide. These carriers were used in enzyme immunoassay instead of polystyrenes. Some advantages of both heterogeneous and homogeneous immunoassays were achieved. HBsAg was assayed to examine the sensitivity of these methods. A significant difference from the negative serum was obtained for 0.5 μg/L. 43 serum samples were assayed, and there was a good correlation between these methods and ELISA.