Oligonucleotide primers derived from the ipaH7.8 sequence have been used to determine the boundaries of DNA sequence homology among five lpaH genes on the invasion plasmid (pWR100) of Shigella flexneri 5, strain M9OT-W. The primary structure of lpaH4.5 has been established from DNA sequence analysis. The first 197 amino acids in lpaH7.8 were replaced in lpaH4.5 by a unique set of 251 amino acids, generating two related proteins with variable and conserved sequences. The amino-terminal region of lpaH4.5 displayed an internal repeat structure, also seen in lpaH7.8, characteristic of members of the leucine-rich glycoprotein (LRG) family. The DNA sequences of ipaH2.5 and ipaH1.4 indicate that these genes are truncated versions of lpaH7.8. Western blot analysis of a λgt11 ipaH recombinant (W7) subclone demonstrated that the antigenicity of lpaH7.8 resides outside the leucine-rich repetitive region.