In a sandwich ELISA for tetanus antibodies, the influence of the tetanus toxoid concentration used for coating microtiter plates has been studied. The antibody levels by toxin neutralization bioassay and by ELISA were studied for a population with known immunization history. By decreasing the tetanus toxoid concentration in ELISA from 5 to 0.2 Lf/ml, a better correlation was found between the ELISA results and the bioassay titers, but sera from recently immunized people still showed high ELISA titers. This phenomenon cannot be ascribed to nonspecific reactions since sera from nonimmunized people are negative in both assays. All sera negative in ELISA had, however, a bioassay titer beneath 0.01 IU/ml.