In vitro methods are used as an alternative to the expensive and time-consuming official method in mice for the titration of tetanus antibodies. Numerous techniques have been developed and used for this purpose, but with mixed results. The difficulty is to get good correlation with in vivo units whatever the vaccinal status of the individual. Today, some of these serological techniques have been virtually abandoned in favor of others which are becoming widespread. RIA is still used, but only in laboratories which already have the necessary equipment and skill. It is gradually replaced by ELISA, a rather delicate technique which however has kept its promises and is amenable to further progress. Passive haemagglutination has been greatly improved by the use of turkey erythrocytes. With this modification, this technique becomes reliable and sensitive and is apparently able to give good results for a large number of sera in case of mass investigation as well as for a quick estimate of the protective level in an individual.