Immunity following diphtheria vaccination in childhood is temporary, and recent outbreaks of diphtheria in adult populations evoked interest in the effect of and side-reactions to revaccination of adults. 237 military recruits were randomly allocated to revaccination with 6 Lf tetanus toxoid or 6 Lf tetanus toxoid combined with 2 Lf or 5 Lf diphtheria toxoid. Side-reactions were recorded one week later, and antitoxin response was assessed after 4 weeks. Protective serum diphtheria antitoxin levels were attained by all subjects receiving diphtheria toxoid containing vaccines. Antibody response was related to dose, indicating a safer long-term protection by revaccination with 5 Lf diphtheria toxoid. All vaccinees, except one without documentation for primary vaccination, attained high tetanus antitoxin levels. Interference phenomena between toxoids were insignificant. Mild local reactions were reported by 22% of the vaccinees. More pronounced local reactions were experienced by 5% and systemic reactions by 3%, independent of vaccine. No serious reactions were observed. Reactions were significantly related to tetanus antitoxin response only. It was concluded that combined revaccination of adults, primary vaccinated around 20 years previously, may be performed without immune assessments.