Members of the Burkholderia cepacia complex (BCC) of organisms are important opportunistic pathogens, particularly in cystic fibrosis (CF) patients. Infection with members of the BCC is associated with poor prognosis. There is speculation that the BCC are intracellular pathogens, surviving and growing inside epithelial cells or professional phagocytes. B. cenocepacia strains are the predominant Canadian BCC-CF pathogens in that members of this species cause the most numerous infections, are the most highly transmissible, and are associated with the highest rates of systemic illness and mortality. B. multivorans infections, though prevalent, are associated with lower rates of transmissibility and mortality. The apparent differential pathogenic ability of these two species has not yet been studied in detail in any model systems. The purpose of these studies was to determine the mechanisms of differential pathogenicity between these two related bacterial species. Three different models of murine infection were used to evaluate B. multivorans and B. cenocepacia infections: an intraperitoneal model of systemic infection, a leukopenic model of pulmonary infection, and an immunocompetent model of pulmonary infection. Differences in the infection kinetics of B. multivorans and B. cenocepacia were observed in all three systems. These potential differences in the pathogenic capability of B. multivorans and B. cenocepacia were further characterized in the immunocompetent model of infection. In the immunocompetent model of intranasal infection, mice were challenged with a 7 single dose of bacteria in stationary phase that had been adjusted to 10 CFU. B. cenocepacia strain C6433 caused a greater degree of systemic illness in mice despite speedy clearance from the lung; persistent B. multivorans strain C5568 caused no systemic illness in mice. The differential infection kinetics and host toxicity demonstrated in this model mirrored observations in a leukopenic model of pulmonary infection, as well as an intraperitoneal infection model. The pulmonary host response in the high-dose intranasal model was evaluated. Bronchoalveolar lavage fluid (BALF) was collected for cytological profiling and immunoassay for pro-inflammatory cytokines. Effective clearance of B. cenocepacia C6433 infection was associated with a more pronounced interleukin (IL)-lβ induction and neutrophil response, on Day 1 of infection. B. multivorans C5568 infection was associated with a gradual increase in the level of neutrophils in the lung, which peaked on Day 2, and a delayed IL-1β response. There was no difference in pulmonary levels of tumour necrosis factor (TNF)-α and macrophage inflammatory protein (MIP)-2 in response to infection with either strain; challenge with both C5568 and C6433 was associated with a rapid increase of TNF-α and MIP-2, followed by a rapid decline. Bacterial persistence in the lung was also examined in the high-dose intranasal model. Immunofluorescent localization of bacteria in day 4-infected lung tissues detected bacteria in association with Mac-3+ mononuclear cells, likely macrophages, in both alveolar space and in lymphoid aggregates. Transmission electron microscopy showed bacteria in membrane-bound vacuoles of alveolar macrophages of day 4-infected lungs. In vitro experiments with primary alveolar macrophages as well as the murine alveolar macrophage cell line, MH-S, showed that B. multivorans strain C5568 had a greater association index with host macrophages than B. cenocepacia strain C6433. These results suggest that B. multivorans strain C5568 may persist in the mouse by virtue of establishing intracellular infection in host macrophages, while B. cenocepacia strain C6433 may be cleared by eliciting a vigorous host response.