The aim of this study, carried out in 1993, was to evaluate diphtheria immunity in Siena. Diphtheria antitoxin levels were measured by means of the immunoenzymatic test (ELISA) in serum samples of 602 apparently healthy subjects (239 males and 363 females) of all ages residing in Siena. According to widely used criteria, 6% of the total population were susceptible to diphtheria (antibody levels <0·01 IU/ml), 71% had basic protection (0·01–0·09 IU/ml) and 23% were fully protected ([ges ]0·1 IU/ml). The results suggested that a high proportion of young population had a protective level of immunity against diphtheria, that susceptibility increased with age and a smaller proportion of males (2·9%) than females (8·3%) were unprotected; this difference was statistically significant. Our results suggest that it may be useful to revaccinate adults with low levels of diphtheria toxoid so that the percentage that remains unprotected does not put the community at risk of an outbreak of diphtheria.