A restrospective study of the influence of age on the efficacy of primary immunization against pertussis was done in Tulangan district, Sidoarjo, Surabaya. Five hundreds and seventy children under ten months of age with various vaccination status and 157 children under three years of age as a control group were taken as samples. Adjuvanted "whole-cell pertussis-vaccine", made by Bio Farma, was used in the study. One tenth of finger blood or toe blood were taken by heparinized capillary pipet for the examination of agglutinin titres against pertussis. Serological examination by micro-agglutination test, showed there was no significant differences in the antibody production following administration of 1, 2 and 3 shots of DPT vaccine among children of 0-2 months, 3-6 months and 7-10 months. The results suggested that the administration of primary DPT vaccine to babies as early as 4 weeks old is effective and can be recommended.