Objective: To establish safety and immunogenicity of a reformulated whole cell pertussis based diphtheria-tetanus-pertussis vaccine (DTP(w)) at the 18-month booster stage following a 2,4, and 6-month primary immunisation course. Method: Open trial in suburban Melbourne in 100 healthy children initially recruited through maternal and child health centres. Thirty-five subjects were bled prior to vaccination, and 4-6 weeks after vaccination. A 7-day diary card was used to record subject temperatures and other systemic and local clinical signs. Results: The increase in antibody geometric mean titres (GMT) after boosting was 19.6-fold (95%ci 14.2, 27.2) for tetanus and 26.5-fold (95%ci 16.6, 42.4) for diphtheria. Pertussis antibody GMTs also all showed substantial increases following the booster, with mean fold changes in titre ranging from 7.3 (Agg2) to 31.3 (Fha). Seventeen percent of subjects (95%ci 10%, 26%) experienced axillary temperatures ≤38°C during the 24-h period following vaccination. Low rates of significant (>25 mm) injection site redness (13%) and swelling (8%) were recorded at 24 h postvaccination. Conclusion: This vaccine was well tolerated by children at 18 months of age, and showed substantial boosting of antibody to all components.