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To determine the diphtheria and tetanus antitoxin levels among blood donors in Toronto.Cross-sectional seroprevalence study.Two fixed-site blood-donation clinics in Toronto from September to November 1994.Blood donors 20 years of age or older were eligible to participate; of the 781 eligible donors, 710 (90.9%) participated in the study.Diphtheria and tetanus antitoxin levels and factors associated with disease susceptibility, such as vaccination history, country of birth, age and sex. A diphtheria antitoxin level lower than 0.01 IU/mL and a tetanus antitoxin level lower than 0.15 IU/mL were considered nonprotective.Among the participants, 147 (20.7%) had a diphtheria antitoxin level in the nonprotective range, and 124 (17.5%) had a tetanus antitoxin level that was nonprotective. Increasing age and lack of written vaccination records were associated with susceptibility to the 2 diseases. Birth outside Canada was significantly related to tetanus susceptibility.Adults over 50 years of age who did not know their vaccination history were the least likely to be protected against diphtheria and tetanus. The greatest benefit of any immunization strategy would be gained by targeting this group.