The prevalence of bacterial antibodies was determined in 173 children aged 0–15 years. The prevalence of IgGBorrelia burgdorferiantibodies in titres>500 in children less than 8 years of age was 6% while none of the older children had these antibodies in titres>400. IgGHelicobacter pyloriantibodies were detected only in children older than 6 years of age, with a prevalence of 6.5%, as were IgAH. pyloriantibodies, with a prevalence of 3.7%. The prevalence of high‐titre IgGCampylobacter jejuniantibodies was 1.2%, that of IgA 1.8% and IgM 1.2%. The prevalence of high‐titre (>500 IU/ml) antistreptolysin O was 3%, that of antistaphylolysin‐alpha (≥ 4 IU/ml) 2% and that of antiteichoic acid antibodies (titre 2) 2%. Low‐titreYersiniaantibodies were detected in 2%. High‐titreBordetella pertussisantibodies were detected in 6% of recently vaccinated children and in 8% of children in their first years of school. In the latter, high‐titre antibodies were mainly of the IgM and IgA classes. Altogether 35 children tested positive for bacterial antibodies other thanBordetella pertussisantibodies. Clinical evaluation revealed a possible infection, suggested by the antibody, in 5 (3%) of the children. Two (vaccinated) children had evidence of whooping cough. Eight of the 35 children with high‐titre bacterial antibodies (23%) also had elevated levels of autoantibodies (but not aut