Plasma-polymerization has recently been used to make an ultrathin film on a variety of sensors and immunoassays. Here, plasma-polymerized films were formed on flat glass plates using allylamine, acrylic acid, acrolein, and allylcyanide as monomers. Adsorption of antibody (IgG class), its fragments (F(ab`)<sub>2</sub> and Fc) and human serum albumin onto these plasmapolymerized films and commercially available polymer plates usually used as substrata for immunoassay was measured. The plasma-polymerized allylamine film adsorbed a greater amount of the F(ab`)₂ fragments than other plasma-polymerized films and polymer plates. In addition, this fragment was most highly adsorbed onto the plasma-polymerized allylamine film among all the proteins examined. In general, the cationic surface of the plasma-polymerized films had higher affinity for protein adsorption than an anionic surface. The F(ab`)<sub>2</sub> fragment took a close-packed “end-on” orientation on plasma-polymerized allylamine film, which desorbed little of the F(ab`)₂ fragment and exchanged only a slight amount of this protein for other proteins. The film was optically transparent, flat and smooth, and could be used successfully as a solid phase in two-site immunoradiometric assay of human IgG.