B. cepacia and B. fragilis exhibited a greater biological activity than previously recognised both in terms of endotoxicity and cytokine induction. For B. cepacia the capacity to induce the proinflammatory cytokines TNF-α and IL-8 from several cell types was significantly, and unexpectedly, higher compared to the other major CF pathogen, Pseudomonas aeruginosa. This enhanced inflammatory potential of B. cepacia was not due to a more efficient LPS signalling pathway. As both CF pathogens appeared to induce TNF-α in a similar manner, the combined effect of both species was examined. Surprisingly, when P. aeruginosa was present in increasing amounts compared to B. cepacia, cytokine levels were down-regulated. These results indicate that B. cepacia has a major potential to cause immune-mediated damage and concurrent colonisation with P. aeruginosa may modulate this effect. For B. fragilis cytokine levels were compared to Escherichia coli, a facultative anaerobic commensal considered of great importance in gut-derived sepsis due to its extremely active LPS. TNF-α levels induced by B. fragilis were 20-fold lower than E. coli. However, considering the predominance of Bacteriodes species in the gut, outnumbering facultative organisms by 100-1000 fold, results imply that as a population B. fragilis may possess as much biological potential as E. coli. Thus, B. fragilis may play a vital role in gut-derived sepsis. The relevance of these findings to the understanding of B. cepacia in CF and of B. fragilis in sepsis is discussed.