Previous studies have provided evidence that an anaerobic bacterium, which degrades dietary oxalate to CO2 and formate, is present in colonic contents of a number of herbivorous species, laboratory rodents and humans. The present study examines the possibility that these bacteria degrade significant amounts of oxalate and can influence colonic oxalate absorption. Guinea pigs adapted to a diet containing 2% sodium oxalate or fed a normal diet were challenged with 67, 135, 170 or 200 mg of sodium oxalate containing 0.5 microCi of [14C]oxalate, which was injected into the cecum. Adapted animals excreted approximately 2% of the 14C in the urine, regardless of the dose, whereas unadapted animals excreted significantly higher amounts in the urine at the two lower doses and died at the two higher doses. Conversely, antibiotic treatment of adapted guinea pigs reduced the ability of their cecal flora to degrade oxalate, and a correspondingly greater percentage of an injected oxalate load was excreted in the urine. Oxalate degradation rates in cecal fluid were depressed by the secondary bile salt deoxycholate, and in vitro studies with pure isolates of guinea pig and human strains of oxalate degraders confirmed that these bacteria were highly sensitive to low concentrations of deoxycholate. Results indicate that these bacteria may be important in preventing excess absorption of oxalate and raise the possibility that the hyperoxaluria associated with bile salt malabsorption of ileal disease in part may be due to suppression of these bacteria by the bile salts.