Microorganisms belonging to the genus Brucella can infect humans and many species of animals. Virulence of the brucellae is thought to be essentially due to their capacity to survive and replicate within the phagocytic cells. However, many gaps remain in our understanding of this ability of brucellae to elude the bactericidal effects of host phagocytes, and basic questions remain unanswered. Identification of Brucella gene products which are related to intracellular survival, as well as those which contribute to the induction of protective immunity, is critical to elucidate the molecular mechanisms of the pathogenesis of the organism. The present article summarizes the current status of the research on gene products and other structural or metabolic factors associated with virulence of the brucellae.