The aim of this study was to evaluate the existence and the rate of pertussis infection in December 2002 among the symptomatic contacts in Konurhaciobasi village of Kirikkale province (placed in middle Anatolia, Turkey, with a population of 500 people), where a laboratory-confirmed pertussis case was defined by culture positivity. Sixty contacts with cough history of more than two weeks have been interviewed and their nasopharyngeal swabs have been collected together with the serum samples from 47 of them. "Probable case" definition included cough lasting longer than two weeks together with at least one of the following symptoms: paroxysms of cough, inspiratory "whooping" and post-tussive vomiting without other apparent cause. The samples were inoculated onto cephalexin Bordet-Gengou agar (C-BGA), and following the growth of suspected colonies identification tests including slide agglutination by using Bordetella pertussis phase 1 and B.parapertussis specific antisera, were performed. IgG antibodies against B.pertussis were searched with an in-house ELISA method by using purified pertussis toxin, and titers of > or = 100 ELISA units (EU)/ml were considered as acute/recent infection. Among the subjects 80% were children and 75% were female. Thirty-six subjects (76.6%) had a history of immunization with 3-4 doses of diphtheria-pertussis-tetanus (DPT) vaccine. According to questionnaires the mean household number was calculated as 5.0 +/- 0.26. Fifteen contacts (age range: 4-12 years, 11 were female) were defined as "probable cases", however all of them were found to be culture negative. All these cases had been vaccinated with DPT vaccine. Among the probable cases 12 serum samples were available, and six of them were evaluated as acute/recent pertussis infection, since their antibody titers were found as > or = 100 EU/ml. In addition, the evaluation of ELISA results of 47 individuals indicated that 27 of them had antibody titers of > or = 100 EU/ml, who could be considered as acute or recent pertussis infection. The data of this field survey indicated a possible pertussis outbreak in this village.