Diphtheria may occur even among previously vaccinated persons and knowledge of the duration of immunity is of crucial importance when designing effective vaccination programmes. In a follow-up study of 42 representative probands revaccinated 8 years previously, a continuous fall-off in antitoxic immunity was demonstrated. 98% were still protected (antitoxin concentration > 0.01 IU/ml). From the distribution of titres in the group the individual risk of susceptibility 8 years after revaccination was calculated to be 0.8/1000 (0.2-2.9/1000, 95% confidence limits). Thus, repeated revaccinations are required to secure continuous protection. The fall-off pattern for diphtheria antitoxin was approximately the same as for tetanus antitoxin. Peak values following revaccination are decisive for the duration of immunity. As peak values following vaccination depend on naturally acquired immunity and consequently decrease as indigenous diphtheria in a population disappears, highly potent vaccines are required to secure long-term immunity following diphtheria revaccination. The effects of dose and adjuvant are discussed.