A seroepidemiological study was conducted in 1994 on a representative sample of the population of Florence in order to verify the immunity coverage against diphtheria. Subjects were divided according to sex and age class. Sera from each selected class were at least 1.5/1000 of the residing population. Diphtheria antitoxin was titrated using a quantitative ELISA test. The results show an overall adjusted prevalence of diphtheria immunity (> or = 0.01 IU/ml) equal to 63.7%. Subjects of younger age classes have good protection levels (85.5% immune under 30 years), while only half individuals aged > or = 50 years have antibody titres > or = 0.01 IU/ml. Full protection (antibody titre > or = 0.1 IU/ml) was detected only in a very small proportion of those aged > or = 40 years. Our data show (1) how a recrudescence of diphtheria could theoretically take place in older subjects living in Italy, and (2) stress the importance of periodical re-vaccination of adults.