In 24 military recruits vaccinated against tetanus 17-20 years earlier antibody responses were studied 4, 11, and 24 days after revaccination. Antitoxin concentration in serum showed a sigmoid increase with titres at day 4 only just above protective level, corresponding to a risk of susceptibility to be expected even 4 days after revaccination (2.0%, 95% confidence limit). Only after 7 days the risk of unprotection was less than 1%. As the incubation period of tetanus can be as short as 24 hours, revaccination only may not be regarded sufficient if more than 17-20 years have elapsed since primary vaccination and conditions favoring rapid development of tetanus are present. The findings may explain some of the cases of tetanus which have occurred in spite of primary vaccination supplemented by emergency booster. The statement that prophylactic tetanus immunoglobulin is never indicated to persons completely vaccinated earlier should be considered for revision.