A collaborative study has been performed to validate two in vitro serological assays, ELISA and ToBI test, as alternatives to the direct challenge procedure for potency testing of tetanus toxoid vaccines for human use (Ph.Eur. monograph Tetanus vaccine (adsorbed) (0452)). In six laboratories, guinea-pigs were immunised with tetanus toxoid vaccines from different manufacturers representing various types of combined products, including one product of borderline quality. Blood samples were taken two to four days before challenge. Parameters that were analysed included: (i) correlation of vaccine potencies obtained by direct challenge test and by serological assays, (ii) prediction of survival based on antibody concentrations, and (iii) correlation of antibody concentrations obtained in ELISA, ToBI test and in vivo Toxin Neutralisation test. In addition, ELISA and ToBI test repeatability and reproducibility were further studied by titration of a total of 28 serum samples in 23 laboratories. This paper provides background information, gives an outline of the experimental design and discusses the study results. It is concluded that ELISA and ToBI test are valid alternatives to the challenge procedure. Implementation of the serological assays as alternatives to the challenge procedure for batch release of tetanus vaccines for human use will result in a marked refinement as well as a substantial reduction of numbers of laboratory animals.