A procedure for covalent immobilization of functional proteins on silica substrates was developed using thiol-terminal silanes and heterobifunctional cross-linkers. Using this procedure, a high density of functional antibodies was bound to glass cover slips and silica fibers. The amount of anti-IgG antibody immobilized was determined to be in the range of 0.66 to 0.96 ng/mm2 using radiolabeled antibody. The relative amount of IgG antigen bound by the immobilized antibody (0.37 to 0.55 mol antigen/mol antibody) was three to five times greater than other investigators have reported. In addition, the amount of protein nonspecifically adsorbed to the antibody-coated surface was further reduced by the addition of blocking agents so that nonspecific adsorption of protein antigens represented only 2-6% of the total antigen binding. With this low background, IgG antigen binding could be measured at levels as low as 150 fmol when an antigen concentration of 3 pmol/ml was applied. The process for antibody immobilization is straightforward, easy to perform, and adaptable for modifying mass quantities of biosensor components.