Tetanus booster is a routine procedure of tetanus prevention in populations with high risk of injury, independent of the levels of protection. But the immune response in already protected individuals is not well studied. We describe the kinetics of booster response in individuals by measuring tetanus antitoxin levels by indirect ELISA. A 6-month follow up was performed on 60 boosted individuals tested before, 1 week, 1, 2, 3 and 6 months after the booster. High initial protection (mean titer 1.08 IU/ml) and less than 3-fold increase after 1 month were observed. After 1 month of stable antitoxin levels, the levels slowly decreased and reached a mean titer of 1.78 IU/ml after 6 months. Individuals with initial levels <1 IU/ml had booster response after the first month twice as high compared to those with initial level >or=1 IU/ml. However, in both groups, the decline from 1 to 6 months was about 2-fold. Individuals already protected against tetanus exhibited an attenuated, short-lasting booster response to tetanus toxoid. This was more pronounced in individuals with pre-booster levels >or=1 IU/ml, who did not improve immune protection after the booster.