The objective of this study is to evaluate the serological response against pertussis after completion of both 2 and 3 doses of DPT vaccination. The study has been carried out retrospectively among 766 children under 3 years of age in Tulang-an District, Sidoarjo, East Java. The antibody titres against pertussis were measured by micro agglutination test. The results showed that the percentage of children having antibody titre of 1 : 80 or more, at 1—5 months post vaccination were 80.9% and 88.3% for 2 and 3 doses, respectively. The results do not differ significantly. This insignificance was maintained up to 17 months of the post-vaccination period.