The autotransporters are a family of extracellular proteins, found in various Gram-negative bacteria, that have many different functions but appear to have a similar mechanism of export. In B. pertussis, the virulence-regulated proteins Pertactin, BrkA, Tcf, and Vag8 have structural homology at their C-termini (30-kDa) and the N-terminal of the mature proteins share structural characteristics such as RGD and SGXG motifs. Recently, another member of the B. pertussis autotransporter family, Bap-5 (Blackburn, 2000) (GenBank accession number AF081494) or BapC (GenBank accession number AJ277634.1) was identified. The present work has suggested that BapC, like BrkA, is a serum-resistance factor. B. pertussis brkA, bapC double and bapC single mutants were created, and showed greater sensitivity to killing by normal human serum than their wild-type strains but they were not as sensitive as a bvg mutant strain. Competition assays also showed an important role for BapC, like BrkA, in virulence of B. pertussis strains after intranasal infection in the mouse. Moreover, the brkA, bapC double and bapC single mutants were found to be more sensitive to the antimicrobial peptide, cecropin P1, than the parent strain. Nucleotide and amino acid analyses of the bapC region spanning the poly(C) and poly(G) tracts of a number of B. pertussis strains showed minor nucleotide and amino acid polymorphisms in some strains but it appeared that all had an ORF that would be able to produce some form of BapC.