Enzyme immobilization onto silicon substrates has been investigated by five different coupling procedures. The methods included covalent coupling either through a metal link reagent or silane reagents containing pendant amino or epoxide linkers, an entrapment technique using a thin layer of gelatin, or an adsorption technique using poly-L-lysine. These immobilization procedures were evaluated using glucose oxidase and a simple spectrophotometric method employing Fenton's reagent. Retention of enzyme activity and surface loading were assessed. The immobilization techniques were also evaluated by electron microscopy to characterize the evenness of the surface coatings. All of the covalent coupling procedures led to surface loadings, approaching I pmol mm(-2); however, the surfaces appeared irregular on a microscopic scale. The poly-L-lysine adsorption technique provided the smoothest surface. With the exception of the entrapment technique, all immobilization procedures provided immobilized enzyme that retained >75% activity after several weeks of storage.