The prevalence of bacterial antibodies was determined in 173 children aged 0-15 years. The prevalence of IgG Borrelia burgdorferi antibodies 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. IgG Helicobacter pylori antibodies were detected only in children older than 6 years of age, with a prevalence of 6.5%, as were IgA H. pylori antibodies, with a prevalence of 3.7%. The prevalence of high-titre IgG Campylobacter jejuni antibodies 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 (> or = 4 IU/ml) 2% and that of anti-teichoic acid antibodies (titre 2) 2%. Low-titre Yersinia antibodies were detected in 2%. High-titre Bordetella pertussis antibodies 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 than Bordetella pertussis antibodies. 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 autoimmune diseases).