Using microwell plates functionalized with surface hydrazide groups, we site-specifically immobilized antibodies to the surface of the wells. The procedure for site-specific, oriented immobilization of IgG to hydrazide surfaces is simple. It requires a brief, mild oxidation of the carbohydrate side chains of IgG with NaIO4 at a pH of approximately 5 and incubation of the oxidized IgG with the hydrazide surface. These oxidized immunoglobulins bind to the hydrazide groups of these plates preferentially by the Fc region of the molecule, resulting in improved specific activity. Enzyme immunoassays for horseradish peroxidase and human IgG were tested on both standard (hydrophobic) and hydrazide group-carrying surfaces. The improved antigen binding capacity of the hydrazide-modified plates results in greatly increased sensitivity (approximately 4 x) and improved linearity in both assays when compared to native, nonoxidized antibody used with standard microwell plates. Furthermore, the hydrazide functionalized plates exhibited lower non specific binding.