The conditioned, passive hemagglutination method, which is simple, quick, economical and very sensitive, is suited for large-scale studies in which it is not possible to carry out the in vivo seroneutralization. However, our experience has confirmed that it has certain drawbacks and restrictions: (1) the technique must be applied very carefully and good laboratory training is required. The results obtained by this method vary according to several factors, especially the degree to which the antigen has been purified and its blood carriers, therefore these results are not always consistent; (2) the antibodies found were not a good indication of the degree of protection except at high titers. A study of correlations between HA titers and neutralizing (mice) titers carried out on the basis of 509 double titrations has demonstrated that, whereas they are satisfactory in clearly immune subjects, they show only mediocre results in subjects who are receptive or who are displaying a primary response: a wide range of variation has been observed as well as an optimalization of HA titers, perhaps because of IgM's. Therefore, the HA method, despite its usefulness, cannot provide precise evaluations for: --the amount of protection provided by an anti-tetanus vaccine, --the proportion of protected subjects in a certain group of people, --an injured person's anti-tetanus immunity. We should work to develop in vitro tests which are both sensitive and reliable in terms of the anti-tetanus protection threshold.