The immunogenicity and reactogenicity of the tetanus-diphtheria adult type vaccine was compared in two groups: group I (n = 201, 18-30 years old, presumably vaccinated with the DTP vaccine) and group II (n = 147, > or = 45 years old, without vaccination antecedents). Before vaccination, the seroprotection levels for tetanus were 90.5% (group I) and 30.6% (group II). These rose to 99.5% and 81.7%, respectively, after administration of one vaccine dose. For diphtheria, prevaccination seroprotection levels were 38.3% (group I) and 19.0% (group II). These rose to 85.8% and 65.7%, respectively, after vaccination. The logistic regression analysis showed an association between antibody titre and age. In group II, 3 doses of Td vaccine were needed to reach titres similar to those achieved in group I with a single dose. Stated reactogenicity was greater in: young subjects, women, those with higher titres of tetanus antibodies and those receiving other vaccines simultaneously. These results confirm the need for vaccination schedules adapted to the characteristics of each population age-group.