To increase the sensitivity of immunoassay or immunosensor, many amounts of antibody should be immobilized. Allylamine thin film was formed on a flat silver plate by plasma-polymerization and this plate was referred to as Ag(ALAM). Adsorption of F(ab')2anti-human IgG onto Ag and Ag(ALAM) was investigated. The following results were obtained: (1) The adsorption isotherm of F(ab')2anti-hIgG onto Ag(ALAM) or Ag was a Langmuir type. The saturation binding for this antibody onto Ag(ALAM) was approximately 2-fold than that on Ag. (2) Ag(ALAM) and Ag were used as solid phases in two-site immunoradiometric assay (IRMA) of human serum IgG, and dose responses were compared. The dose response on Ag(ALAM) occurred at lower concentration than that on Ag and the magnitude of the dose response on Ag(ALAM) was larger than that on Ag.