Cheddar cheese was evaluated as a food carrier for the delivery of viable microorganisms of Enterococcus faecium (Fargo 688; Quest Int., Naarden, The Netherlands) to the gastrointestinal tract. This strain had previously been shown to possess properties required of a probiotic microorganism including the ability to relieve irritable bowel syndrome. The strain was found to survive to high numbers in Cheddar cheese during ripening at 8 degrees C for 15 mo (4 x 10(8) cfu/g) and in yogurt during storage at 4 degrees C for 21 d (4 x 10(7) cfu/g). In an in vitro model system, Cheddar cheese was found to have a greater protective effect than yogurt upon exposure of the probiotic culture to porcine gastric juice at pH 2. Subsequently, a feeding trial involving 8 pigs per group was performed in which a rifampicin-resistant variant of the probiotic strain was fed for 21 d at a mean daily intake of 1.3 x 10(10) cfu/d from Cheddar cheese or 3.7 x 10(9) cfu/d from yogurt. During the feeding period, Cheddar cheese yielded a significantly higher mean fecal probiotic count (2 x 10(6) cfu/g of feces) than did yogurt (5.2 x 10(5) cfu/g of feces). These data indicate that mature Cheddar cheese compares very favorably with fresh yogurt as a delivery system for viable probiotic microorganisms to the gastrointestinal tract.