Five patients with blind loop syndrome (Billroth II) were examined by measuring 14CO2 specific activity of expired breath samples taken at intervals after a meal containing glycine-1-14C cholate. The 5 patients tested showed a marked increase of 14CO2 specific activity. Furthermore, the ability of deconjugation of bacteria isolated from the jejunal fluids in the efferent loop of these patients was tested by thin layer chromatography. The bacterial species identified from the samples were as follows: enterococcus, Lactobacillus (L) buchneri, L. bifidus, L. brevis, Eubacterium (E) lentum, Bacteroides (B) vulgaricus, B. filamentosum, Corynebacterium (C) granulosum, Escherichia (E) coli, Staphylococcus (S) epidermidis, and Aerobacter (A) aerogenes. These species of bacteria, except E. coli and A. aerogenes, showed the deconjugation ability by which conjugated bile acids in ox gall was hydrolyzed. Administration of chloramphenicol (1g per day for 14 days orally divided doses) to the 5 patients reduced 14CO2 specific activity significantly. On the other hand, 9 healthy men (control subjects) who were tested showed a flat curve, and 8 of the 9 had no growth of bacteria isolated from the jejunal fluids. The remaining healthy man showed an overgrowth of E. coli and Pseudomonas (P) aeruginosa, but the species did not have the ability of deconjugation. Thus, we concluded that the patients with blind loop syndrome(Billroth II) had the bacterial overgrowth in the efferent loop that contained species with deconjugation ability, and, as a result the bacterial overgrowth contributed to causing abnormalities (increased deconjugation) in the metabolism of bile acids in the small intestine. When the concentration of conjugated bile acids in the small intestine was reduced to levels below the critical micellar concentration by several factors, fat malabsorption and subsequent steatorrhea were induced (1,-4). Furthermore, H. Fromm and A. F. Hofmann presented in vivo that the patients with blind loop syndrome had fat malabsorption and the patients who had a high 14CO2 output after oral administration of glycine-1-14C cholate showed a low 14CO2 output after oral administration of antibiotic drug (5,6). However, there has been no report on the deconjugation ability of bile acids of bacteria isolated from the jejunal fluids in the efferent loop of patients with Billroth II who had positive breath tests.(ABSTRACT TRUNCATED AT 400 WORDS)