The immune status against tetanus in relation to vaccination was investigated among healthy populations in 3 selected provinces in Turkey (n=2094). In-house ELISA supplemented with the particle agglutination test was performed for this purpose. An exact correlation was found between vaccination status and immunity against tetanus. The immunity was high among children and sharply decreased with age among adults. The primary vaccination for children increased the immunity, reaching a geometric mean titer (GMT) of 6.2 IU/ml at the fourth dose. There was a reduction of immunity during the next y, followed by an increase with a booster injection at primary school age. Among adolescents and adults, the GMTs after the last vaccination fell off exponentially with a slope of -0.068 log10 IU/ml per y. Extrapolation of the regression line predicted that the minimum protective level (> or =0.01 IU/ml) would be maintained for approximately 30 y on average after the last vaccination. Diyarbakir, 1 of the selected provinces, had lower immunity than the other 2 provinces with poor immunization. Adult females tended to show slightly higher GMTs than males, probably due to the neonatal tetanus elimination program. Reinforcement of immunization against tetanus for adults is recommended.