OBJECTIVE Despite extensive childhood immunization, pertussis remains one of the world's leading causes of vaccine-preventable deaths. Incidence of pertussis in adolescents and adults has increased in many countries despite high vaccination coverage. In China, booster vaccinations against diphtheria, tetanus and pertussis are not used in adults, and little is known about pertussis incidence in the age group. The aim of this study was to determine seroprevalence of IgG antibodies to pertussis toxin (PT) and diphtheria among adults in China. METHODS Blood samples were obtained from 210 healthy adults aged 18-50 years in Weifang city, China during the period of May and June 2010. Serum IgG antibodies against PT (anti-PT IgG) and diphtheria were determined by the commercial ELISA kits, respectively. According to the kit, concentration of anti-PT IgG higher than 30 IU/mL was considered positive. An antibody concentration of ≥ 0.1 IU/mL was defined as evidence of seroprotection against diphtheria. RESULTS The mean concentration of anti-PT IgG antibodies was 9.95 IU/mL (95% confidence interval (CI) 8.45-11.44). Eleven (5.24%) of the studied subjects were proved to be seropositive to pertussis. Of the 210 subjects, 161 (76.6%) had anti-diphtheria antibody concentration ≥ 0.1 IU/mL and 49 (23.3%) had the antibody concentration between 0.01 and 0.099 IU/mL. CONCLUSIONS Our study indicated that about 5% of adults aged 18-50 years had positive anti-PT IgG antibodies, suggesting that adult pertussis is not uncommon in China. Although a high proportion of studied subjects had a protective level of immunity against diphtheria, the antibody level decreased with the increasing age of adults. Booster vaccinations against pertussis should be considered in adults in China.