In Denmark, childhood primary vaccination against diphtheria and tetanus has been recommended since 1950. No routine revaccinations or general vaccination of adults have been offered. In most other western countries revaccinations are recommended later in childhood. However, death still occurs from diphtheria and tetanus in countries with such vaccination programs although it is generally accepted that protective immunity can be obtained by vaccination. On this basis the immunity against diphtheria and tetanus was assessed in a random sample of 351 subjects in the age range 30-70 years. Diphtheria antitoxin titres were determined by in vitro neutralization technique using VERO cells. 26% had diphtheria antitoxin titres below protective level (0.01 international antitoxin units/ml serum). The highest number of unprotected against diphtheria was found among 30-39 year old women (68%) and 60-69 year old subjects (36%). Tetanus titres were determined by a combination of ELISA technique and an in vivo neutralization assay. 51% were unprotected against tetanus (less than 0.01 international antitoxin units/ml serum). The highest number of unprotected against tetanus was found among 60-69 year old subjects (68%) and especially among females in this age group (77%). To avoid epidemics of diphtheria in the future the immunity among adults must be raised within the coming years. Thus, revaccination must be recommended and high attendance ensured. One revaccination is sufficient only when complete primary vaccination is documented.