Accurate determination of the concentrations of immunoglobulin G (IgG) antibody to tetanus toxoid is important in order to evaluate the immunogenicity of tetanus toxoid vaccines, determine immune competence in individual patients, and measure the prevalence of immunity in populations. The performance of three commercially available enzyme-linked immunosorbent assays (ELISAs) for IgG antibodies to tetanus toxoid were evaluated. Serially diluted NIBSC 76/589 and TE-3 human tetanus IgG immunoglobulin international reference standards were analyzed in quadruplicate using ELISAs manufactured by The Binding Site, Inc. (VaccZyme); Scimedx; and Euroimmun. In addition, IgG antibodies to tetanus toxoid were measured in 83 deidentified serum specimens using each manufacturer's ELISA. Each ELISA provided linear results when evaluated with the reference preparations. The Binding Site ELISA provided results that closely corresponded to the reference preparations (y=1.09x-0.08), whereas the Scimedx ELISA gave results that were consistently lower (y=0.21x-0.07) and the Euroimmun ELISA gave results that were consistently higher (y=1.5x+0.30) than the reference preparation concentrations. Using the recommended cutoff for each ELISA (<0.10 IU/ml), the overall agreement of all of the ELISA methods was 78%. Three of eighty-three (3.6%) human serum samples demonstrated inadequate immunity with all three assays. The Binding Site ELISA yielded nonprotective antibody concentrations in only these 3 samples, whereas 19 samples (22.9%) according to the Scimedx ELISA and 6 samples (7.2%) according to the Euroimmun ELISA demonstrated nonprotective concentrations. The performance characteristics of ELISAs for tetanus immunoglobulin titers were manufacturer dependent, and the differences translated into important disparities in reported results.