The effect of microemulsion as reaction medium for immobilization of proteins to polystyrene hydrophilized with poly(ethylene glycol) or polysaccharide has been investigated. The amount of albumin, immunoglobulin G (IgG) and collagen immobilized from microemulsion was increased 10-100 times, compared with immobilization from an aqueous buffer system. The stability of anti-IgG and interleukin-2 was studied in different microemulsion systems. Anti-IgG was found to be stable in microemulsions based on sodium bis(2-ethylhexyl)sulphosuccinate (AOT) and C12EO5, whereas the non-ionic surfactant di-C9EO9 almost instantly destroyed the antigen-binding properties. Interleukin-2 completely lost its biological activity in an AOT-based microemulsion. We have also found that the choice of microemulsion influences coupling efficiency of the protein. An AOT-based system is preferred for immobilization of mycoplasma antibody, while a microemulsion based on C12EO5 is preferred for borrelia antigen.