The major outer membrane proteins (OMPs) of Brucella spp. were initially identified in the early 1980s by selective extraction techniques and classified according to their apparent molecular mass as 36-38 kDa OMPs or group 2 porin proteins and 31-34 kDa and 25-27 kDa OMPs which belong to the group 3 proteins. Variation in apparent molecular mass is essentially due to association with peptidoglycan subunits of different sizes. Two genes, omp2a and omp2b, which are closely linked in the Brucella genome, and which share a great degree of homology (> 85%), encode the 36 kDa porin proteins, now named Omp2a and Omp2b proteins respectively. Two genes code for the group 3 OMPs and are named omp25 and omp31. The predicted amino acid sequences of omp25 and omp31 share 34% identity. Furthermore, all Brucella major OMPs share amino acid sequence homology with the major OMPs RopA or RopB of Rhizobium leguminosarum, which supports the close genetic relationship of brucellae with members of the alpha-2 subdivision of the class Proteobacteria. Another characteristic common to the major OMPs of R. leguminosarum and Brucella is that they are tightly, probably covalently, associated with the peptidoglycan. The major OMP genes display diversity among Brucella species, biovars and strains allowing their differentiation, and the polymorphic markers identified have brought new insights into the evolutionary development of the genus Brucella, antigenic variability of brucellae, and future prospects in the field of vaccine development.