Studies of diphtheria antitoxin levels in serum from adult populations have indicated high frequencies of unprotected subjects. Serum from 351 randomly selected Danes between 30 and 70 years old has been assessed for antitoxin concentration; 123 persons among these, who had low antibody levels, received one vaccination with 2 Lf, 5 Lf or 12 Lf diphtheria toxoid. Side-reactions were recorded, and antibody levels were studied 4 weeks later. Antitoxin concentration following vaccination increased markedly to above protective level in 83% of those vaccinated. Of subjects, who could document a complete primary vaccination series, only one receiving 2 Lf 31 years after primary vaccination did not attain protective antitoxin level. Minor local reactions only were recorded among subjects who did not respond serologically to vaccination. Frequencies of more pronounced reactions experienced by serologically responding subjects depended on dose and were 15%, 14% and 23% respectively. It was concluded that a single vaccination of the present adult Danish population will induce protective antibody levels so frequently that the effect of herd immunity will secure against epidemics if future diphtheria outbreaks are experienced. To secure individual protection, vaccination history and/or serological assessments have to be explored in order to decide whether a single vaccination is sufficient.