Brucella abortusis an intracellular pathogen of monocytes, macrophages, dendritic cells, and placental trophoblasts. This bacterium causes a chronic disease in bovines and in humans. In these hosts, the bacterium also invades neutrophils; however, it fails to replicate and just resists the killing action of these leukocytes without inducing significant activation or neutrophilia. Moreover,B. abortuscauses the premature cell death of human neutrophils. In the murine model, the bacterium is found within macrophages and dendritic cells at early times of infection but seldom in neutrophils. Based on this observation, we explored the interaction of mouse neutrophils withB. abortus. In contrast to human, dog, and bovine neutrophils, naive mouse neutrophils fail to recognize smoothB. abortusbacteria at early stages of infection. Murine normal serum components do not opsonize smoothBrucellastrains, and neutrophil phagocytosis is achieved only after the appearance of antibodies. Alternatively, mouse normal serum is capable of opsonizing roughBrucellamutants. Despite this, neutrophils still fail to killBrucella, and the bacterium induces cell death of murine leukocytes. In addition, mouse serum does not opsonizeYersinia enterocoliticaO:9, a bacterium displaying the same surface polysaccharide antigen as smoothB. abortus. Therefore, the lack of murine serum opsonization and absence of murine neutrophil recognition are specific, and the molecules responsible for theBrucellacamouflage areN-formyl-perosamine surface homopolysaccharides. Although the mouse is a valuable model for understanding the immunobiology of brucellosis, direct extrapolation from one animal system to another has to be undertaken with caution.