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According to the specifications of the European Pharmacopoeia (EP) monograph on Tetanus vaccine for veterinary use (EP 1990) potency testing is based either on immunization and bleeding of guinea-pigs or rabbits, followed by estimation of the antitoxin titre by toxin neutralization assay (TN test) in mice (Method A) or on immunization and challenge of mice (Method B). Recently in vitro serological assay systems have been described as potential alternatives to the TN test. These include Enzyme Linked Immunosorbent Assay (ELISA), Toxin Binding Inhibition (ToBI) test, Haemagglutination (HA) test and modifications of these procedures such as antigen capture ELISA and antigen competition ELISA. An interlaboratory validation study was initiated to evaluate the suitability of the in vitro serological methods to provide valid and reproducible measurements of the biological potency by Method A of the EP. This report presents the results of this study. Intra- and interlaboratory variation was acceptable for ELISA and ToBI test, but large variation was seen in the HA test. Estimates of potency obtained by TN test and by in vitro test systems were in good agreement for a wide range of products and a wide range of antitoxin titres. It is concluded that the TN test for the estimation of potency of tetanus toxoid vaccines for veterinary use according to Method A of the EP can be replaced by ELISA or ToBI test. Standardization of HA test is required before this test can be used for quality control purposes.