A fragment of the Salmonella typhimurium virulence plasmid containing the rck locus, when cloned in the recombinant cosmid pADE016, was shown previously to confer high-level complement resistance on both rough and smooth Escherichia coli, Salmonella minnesota, and S. typhimurium and was associated with the production of an outer membrane protein. We determined the nucleotide sequence of the fragment containing the rck locus. Mutations in the two major open reading frames confirmed that the complement resistance mediated by pADE016 was due to a single 555-bp rck gene encoding a 17-kDa outer membrane protein. Analysis of the rck gene revealed that the Rck outer membrane protein consisted of 185 amino acid residues, with a calculated postcleavage molecular mass of 17.4 kDa. Rck is homologous to a family of outer membrane proteins expressed in gram-negative bacteria, two of which have been associated with virulence-related phenotypes: PagC, required by S. typhimurium for survival in macrophages and for virulence in mice; and Ail, a product of the Yersinia enterocolitica chromosome capable of mediating bacterial adherence to and invasion of epithelial cell lines. Rck, most closely related to PagC, represents the third outer membrane protein in this five-member family with a distinct virulence-associated phenotype.