Conjugated linoleic acid (CLA) is produced from linoleic acid (LA) by bacteria in the rumen of herbivores. CLA enters the human diet mainly via milk fat and fatty beef; it acts as an effective anticarcinogen and exhibits other important physiological effects. The objective of the current study was to investigate the capability of a LA-conjugating bacterial community isolated from a human volunteer and associated with germ-free rats to supply the host with CLA. Gnotobiotic rats were fed a diet enriched with esterified LA in the form of sunflower-seed oil. The control group was fed the same diet and remained germ-free. Bacterial cell counts, in vitro LA-conjugation activities, and CLA concentration in feces and in the contents of various intestinal segments were determined. After 10 wk, various tissues were analyzed for CLA concentrations. LA-conjugation activity was found only in feces, cecum and colon content samples from associated rats, but CLA accumulation in various body tissues did not differ significantly between the two groups. The ratio of CLA to LA in feces and in cecal and colonic contents did not differ between groups, indicating that the microorganisms in the cecum and the colon do not synthesize substantial amounts of CLA in vivo and therefore, do not contribute to the CLA supplementation of the host.