A single blinded randomized controlled trial to compare the reactogenicity and immunogenicity of adult formulated dTpa and monovalent pa vaccines with a licensed Td vaccine. Five hundred and forty-eight healthy adults aged 19-70 years received a single injection of dTpa or separate injections of pa or Td (with the alternate vaccine 1 month later). Local and systemic reactions were monitored for 15 days after each vaccination. Serum antibody levels were measured immediately prior to and 1 month after vaccination. Antibody levels were measured 12 months after vaccination in 100 subjects. There was no difference in the total frequency of symptoms and signs between subjects receiving any of the three vaccines. There was a significantly lower incidence of local reactions following pa (60%) than dTpa (80%, P=0.002) or Td (93%, P=0.0008). The incidence of clinically significant (Grade 2 or 3) swelling (> or =20 mm) was higher for Td (20%, P=0.002) than for dTpa (11%) or for pa (2%), however, there were no other significant differences in the incidence of Grade 2 or 3 reactions between the vaccines. A high anti-pertussis seroconversion rate (>97%) against all the studied pertussis antigens was seen 1 month after vaccination with dTpa and pa. A total of 96 and 99% of subjects receiving dTpa and Td, respectively, had anti-diphtheria titres > or =0.01 IU/ml, and all but one subject had anti-tetanus titres > or =0.1 IU/ml after 1 month. Twelve months after vaccination the majority (90-100%) of the subjects were still seropositive for each antigen and although GMTs had decreased they were substantially higher than pre-vaccination levels. The dTpa vaccine was well tolerated and capable of eliciting an immune response against all the antigens in a broad spectrum of the adult population and could potentially replace Td for routine boosters in adults.