The gene coding for the filamentous hemagglutinin (FHA), one of the main factors involved in mediating adherence of Bordetella pertussis to ciliated host cells, was cloned in Escherichia coli, and the 3,500-base-pair nucleotide sequence encoding the amino-terminal region was determined. Molecular cloning, together with the characterization of recombinant FHA-related proteins produced in E. coli, revealed that the primary translation product is a protein of about 370 kilodaltons (kDa). The mature 220-kDa FHA polypeptide secreted by B. pertussis is most probably generated by proteolytic processing that eliminates a carboxy-terminal portion of about 150 kDa. The 1,087 amino-terminal residues of the predicted FHA sequence showed a number of remarkable features. Extensive homology to the Serratia marcescens and Proteus mirabilis hemolysin proteins was found between amino acids 91 and 205 of the FHA sequence, suggesting involvement of this FHA domain in host cell binding or secretion of FHA from B. pertussis. In addition, two regions containing repetitive amino acid sequences were identified. One region, extending from residues 382 to 664, was formed by six repeats, and a second, extending from residues 701 to 912, contained three repeats. The reactivities of several recombinant FHA-derived proteins with a panel of monoclonal antibodies identified at least four epitopes composing an immunoreactive domain present in the carboxy-terminal moiety of the mature FHA.