In Denmark primary vaccination against diphtheria is offered in the 5th, 6th, and 15th month of life with doses of 50 Lf. Only those doing military service are routinely revaccinated (with 121/2 Lf, given once). 403 persons offered primary vaccination 25-30 years ago were screened for diphtheria antitoxin titres by the use of neutralisation and haemagglutination tests. 19% of these (10% of the males and 26% of the females) were unprotected (<0·01 IU/ml). Among those not revaccinated 22% had antitoxin titres below protective level. This accords with the continuing decline of diphtheria antitoxin titre after vaccination. Among those revaccinated against diphtheria in adolescence 5% became unprotected. Thus, persons who were offered primary vaccination against diphtheria 25-30 years ago may be susceptible to diphtheria and its toxic complications. So may those revaccinated more than 10 years ago. Should diphtheria emerge in a community those who received their primary vaccination more than 2 years ago or revaccination more than 10 years ago ought to be revaccinated. Revaccination is also advisable for those travelling to countries with endemic diphtheria. Moreover, since 10% of the present population were unprotected against tetanus it seems advisable to increase the immunity against diphtheria and tetanus by routine revaccination with a combined diphtheria-tetanus vaccine. Only a documented history of vaccinations should be relied on when a decision is being made as to whether to carry out primary vaccination or revaccination.