In this paper we describe the molecular characterization of hrpB, the largest operon in the Xanthomonas campestris pv. vesicatoria hrp cluster. The hrpB region encompasses 6 kb and encodes eight putative proteins, seven of which were expressed in Escherichia coli. The HrpB3 protein is the only one carrying a signal peptide sequence at the N-terminus and is a putative lipoprotein localized in the outer membrane of X. campestris pv. vesicatoria. The HrpB4 and HrpB8 proteins contain one and five putative transmembrane domains, respectively, and are most likely associated with the inner membrane. The HrpB3, HrpB5, HrpB6, and HrpB8 proteins show sequence similarity to putative components of different type III protein secretion pathways in bacteria. Examples include Hrp proteins from other plant pathogens, YscJ, YscN, YscL, and YscT of Yersinia spp., and MxiJ, Spa47, adn Spa29 of Shigella flexneri. The transcription start site and the hrpB promoter was identified. The minimal hrpB promoter region of 90 bp contains a novel sequence motif, the PIP-box, which might play a role in transcription activation of the hrpB operon and possibly other plant-induced genes of X. campestris pv. vesicatoria.