Brucellosis is a worldwide extended zoonosis caused by Brucella spp. These gram-negative bacteria are not readily detected by innate immunity, a virulence-related property largely linked to their surface lipopolysaccharide (LPS). The role of the LPS lipid A and O-polysaccharide in virulence is well known. Moreover, mutation of the glycosyltransferase gene wadC of Brucella abortus, although not affecting O-polysaccharide assembly onto the lipid-A core section causes a core oligosaccharide defect that increases recognition by innate immunity. Here, we report on a second gene (wadB) encoding a LPS core glycosyltransferase not involved in the assembly of the O-polysaccharide-linked core section. As compared to wild-type B. abortus, a wadB mutant was sensitive to bactericidal peptides and non-immune serum, and was attenuated in mice and dendritic cells. These observations show that as WadC, WadB is also involved in the assembly of a branch of Brucella LPS core and support the concept that this LPS section is a virulence-related structure.