Immunity to diphtheria was assessed in serum samples obtained from 3111 healthy Italian males and females aged 0-84 years. Diphtheria antitoxin was tested using a double-antigen, time-resolved fluorescence immunoassay (DA-DELFIA). According to internationally accepted criteria, antitoxin concentrations < 0.01 IU/ml indicate susceptibility to diphtheria, those > or = 0.01-0.09 IU/ml provide basic or inadequate protection, and concentrations > or =0.1 IU/ml are protective. By these criteria, 9.9% (95% CI 8.9 to 11.18) of the participants were susceptible to diphtheria, 30.2% (95% CI, 28.6 to 31.9) had basic protection, and 59.9% (95% CI, 58.1 to 61.6) were protected. The prevalence of unprotected individuals showed an age-related increase, up to the 45-49-year-old age group for females and the 50-54-year-old age group for males (34.9% and 31.3% of individuals, respectively). The prevalence of immunity did not significantly differ in relation to sex in any of the age groups. These results indicate that booster shots should be routinely provided to the adult population in order to maintain a protective level of diphtheria antibodies.