A cloned Bam H1 fragment of genomic Bordetella pertussis DNA which recognized a frequently repeated sequence in the genome of B. pertussis was used as a probe in a DNA hybridization assay for the detection of Bordetella. Extensive studies on cross-reactivity were carried out in standardized strains and in cultured swab specimens from patients without suspected pertussis. Hybridizations of cultured clinical specimens from 142 patients with suspected pertussis were in complete agreement with the standard identification methods. The recovery rate of B. pertussis from nasopharyngeal swabs was less than 50%. Therefore the possibility to detect low numbers of B. pertussis in solution (nasopharyngeal aspirates) was investigated. The detection limit of direct hybridization by dot-blot technique was 5 x 10(3)-10(4) B. pertussis. Culturing bacteria on membranes placed on agar plates prior to hybridization showed that the detection limit could be lowered to 10(4), 10(2), and 10(1) cfu after 1, 2 and 3 days' culture, respectively. DNA hybridization under these conditions was found to be sufficiently sensitive and specific for further evaluation in clinical specimens for diagnosis of pertussis.