The O-antigen of the gram negative bacteria Brucella is composed of an homopolymer of 4,6-dideoxy-4-formamido-alpha-D-mannopyranosyl (or perosamine). Several mAb interact specifically with only the O-antigen of certain Brucella species. Although, many studies show that this specific recognition results mainly from the ratios of alpha 1-2 and alpha 1-3 link between the different Brucella strain perosamine residues, little is known about the mAb recognising this O-antigen. In this paper, we describe the binding profile of five anti-Brucella O-antigen mAb to the LPS of two Brucella strains and a bacteria possessing a nearly identical O-antigen: Yersinia enterocolitica 0:9. We show that the specificity of these five mAb can be correlated to their germ line gene usage. Besides, their relative affinity to the different LPS is correlated to their ability to protect against Brucella infection by passive transfer in a mouse model. The analysis of their 3D structure gives new hypothesis of the epitopes recognised.