We have constructed an expression library of Bordetella pertussis DNA sequences, cloned in Escherichia coli. Random DNA fragments were inserted into the beta-galactosidase gene of bacteriophage lambda gt11 and plaques were screened with antibodies directed against filamentous hemagglutinin. The antigen-positive clone obtained expressed B. pertussis sequences as polypeptides fused to beta-galactosidase. The fusion polypeptides reacted both with antibodies to beta-galactosidase and with antibodies to filamentous hemagglutinin. The positive clone contains B. pertussis DNA sequences as determined by specific hybridization to Bordetella DNA.