AbstractIncrease in the sensitivity of an immunosensor or biosensor requires the immobilization of a large amount of proteins. Good materials must be developed for this protein adsorption. Allylamine thin film was formed on a flat silver plate by plasma polymerization; this plate is referred to as Ag(ALAM). Ag(ALAM) films were characterized by SEM, FT–IR and ESCA. Investigations on adsorption and desorption of F(ab')2 anti‐human IgG (hIgG) on to Ag and Ag(ALAM) revealed the following: (1) The adsorption isotherm of F(ab')2 anti‐hIgG on to AG(ALAM) or Ag was of a Langmuir type. The binding constant (Kb) and saturation binding (Ab) for this protein on to Ag(ALAM) were 8.93 l/mol and 181.8 nmol/m2, respectively, and for Ag, were 7.71 l/mole and 87.9 nmol/m2. (2) The extent of desorption of labeled F(ab')2 anti‐hIgG absorbed on the two plates was the same. (3) Ag(ALAM) and Ag were used as solid phases in a two‐site immunoradiometric assay of human serum IgG. The dose‐response on AG(ALAM) occurred at lower concentration, and was of greater magnitude than that on Ag.