Epidemic diphtheria spread to Ukraine in 1991, where it peaked in 1995 with >5,000 reported cases. To refine epidemic control strategies, immunogenicity of a tetanus-diphtheria toxoids vaccine (Td) containing 2 limits of flocculation (Lf) diphtheria toxoid was evaluated. During a mass vaccination campaign, adults at a clinic in Odessa received one dose of Td. At enrollment, 57.2% of 341 study participants had levels of diphtheria antitoxin (DAT) >/=0.1 IU/mL. Thirty and 180 days after receiving one dose of Td, 91.5% and 84.5% of the participants, respectively, had DAT levels >/=0.1 IU/mL. However, among 40- to 49-year-old participants, only 78.8% and 73.8% had DAT levels >/=0.1 IU/mL at 30 and 180 days, respectively. This study suggests that one dose of 2 Lf diphtheria toxoid is highly effective in raising DAT to protective levels in most adults; however, the study also shows that certain age groups, particularly persons 40-49 and, to a lesser degree, 30-39 years old may require additional doses or a complete three-dose primary vaccination series for optimal protection against diphtheria.