The prn gene encoding the 68 kDa protective outer-membrane protein of Bordetella bronchiseptica (P.68 pertactin) was cloned, sequenced and expressed in Escherichia coli. The gene was isolated by DNA:DNA hybridization experiments using a radioactively-labelled fragment of the homologous prn gene from Bordetella parapertussis. DNA sequence analysis reveals that the gene is capable of encoding a protein with a molecular mass of 93996 Da (P.94); this precursor molecule is processed to form the P.68 antigen on the surface of B. bronchiseptica. Heterologous expression of the full-length gene encoding P.94 in Escherichia coli results in similar processing, with the P.68 antigen targeted to the bacterial outer membrane. Comparison of P.94 with the P.93 and P.95 precursors, encoding homologous proteins from Bordetella pertussis and B. parapertussis, shows a high degree (greater than 90%) of homology. The major differences between all three proteins occur in the number of repeats of the two families (Gly-Gly-Xaa-Xaa-Pro)n and (Pro-Gln-Pro)n of reiterated sequence motifs.