Bacterial translocation (BT) is defined as the passage of indigenous bacteria colonizing the intestine through the epithelial mucosa to the mesenteric lymph nodes and other sites. It can even cause the development of lethal sepsis. BT is being studied extensively, most research being conducted on animal, mostly murine, models. The cause of BT is not well known. Despite advances in antimicrobial therapy, the mortality rate associated with bacteremia is still high, and sepsis can often originate from the patient's own intestinal flora. Consequently, a better understanding of the mechanisms of BT as well as mechanisms operating to prevent bacteria from translocating from the gastrointestinal tract may provide more logical treatment of particular patients. BT may be promoted by immunosuppression (probably one of the most important factors causing the increase in BT), disturbances in the normal ecology of the gastrointestinal tract, allowing some indigenous bacteria to overgrow others, and by mucosal injuries. This article discusses the potential role of various factors responsible for BT and the relationship between BT and neoplastic disease. It also emphasizes the significance of the immune system, which appears to play a role in the ontogeny of gut-associated lymphoid tissue (GALT), including the role of IL-2 (which induces an increase in BT) and the macrophage, which participates in the transport of bacteria from the intestinal lumen to mesenteric lymph nodes. A better understanding of the immunopathology of BT may contribute to the development of novel means of therapy for sepsis and other serious complications of bacterial infection.