The (14)C-glycocholate test, including the measurement of marker corrected faecal (14)C, has been assessed in the following groups of subjects: normal controls (18), patients with diarrhoea not attributable to altered bile acid metabolism (21), patients with diverticula of the small intestine (12), patients with previous resection of ileum and often proximal colon (34), and established ileostomists (10). Patients with diverticular disease had increased breath (14)CO(2) excretion, but normal faecal excretion of (14)C, and this test was more frequently abnormal than the Schilling test. Ileostomists excreted increased amounts of faecal (14)C, even when the ileum was intact and apparently normal. The pattern after resection was complex. Breath (14)C output was normal if the ileal resection was less than 25 cm in length, although some of these patients had increased faecal (14)C excretion if, in addition, at least 15 cm of proximal colon had been resected or by-passed. Longer ileal resections were associated with increased breath and/or faecal (14)C excretion, depending in part on the length of colon resected or by-passed and the 24 hour faecal volume. Fewer than half these patients had both increased breath and faecal excretion of isotope and faecal (14)C alone was occasionally normal with an ileal resection of 50 cm of more. The (14)C-glycocholate test was more frequently abnormal than the Schilling test in this group. The use of faecal marker correction had only a minor impact on the results. These data suggest that, in patients with ileal resection, faecal (14)C, like faecal weight, is determined by the extent of colonic resection as well as by the amount of ileum resected.