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During the screening of a Mycobacterium tuberculosis lambda gt-11 gene library with monoclonal antibodies, we detected a recombinant clone, lambda PH7311, which contained a mycobacterial DNA insert that hybridized specifically with DNA of M. tuberculosis complex strains. Part of this insert was sequenced and used for the development of an M. tuberculosis complex-specific polymerase chain reaction (PCR). Only strains belonging to species of the M. tuberculosis complex group contained an amplifiable fragment of 158 base pairs (bp). This fragment was absent in all strains tested belonging to 15 other mycobacterial species. After amplification by PCR and dot blot hybridization with a digoxigenin-labeled oligonucleotide, the limit of detection of purified genomic M. tuberculosis DNA amounted to a quantity corresponding to 20 bacterial cells. By this technique about 10(3) M. tuberculosis bacteria were detectable in sputum. Using PCR, we were also able to detect M. tuberculosis cells in clinical material such as pleural fluid, bronchial washings, and biopsies, and these results were comparable with those obtained by classical bacterial culture. Of 34 M. tuberculosis strains, 5 did not carry the amplifiable 158-bp fragment, which occurs usually as a single copy in the chromosome. Evidence is presented that the 158-bp fragment is located near a repeated sequence in the chromosome. We presume that strains which did not carry the 158-bp fragment have lost a chromosomal segment by a genetic rearrangement induced by the repetitive DNA element.