23-Selena-25-homotaurocholate is a synthetic bile acid labeled with a gamma-ray-emitting radioisotope 75Se. It is readily measured using external detectors and is thus suitable for whole-body counting. Whole-body retention was measured at 0, 4, and 7 days after oral administration to normal controls and to patients with disease of the small intestine, colon, or ileocecal region. Whole-body retention of less than 25% of the administered radioactivity within 4 days is definitely abnormal, but there was overlap between normal and abnormal groups at this time. At 7 days, whole-body retention less than 12% is abnormal and greater than 19% is normal. Between these limits, values may represent minimal ileal dysfunction not demonstrable by older techniques. Excretion of 23-selena-25-homotaurocholate follows a biexponential curve. The faster component has a half-life similar to that of natural bile acid. It is uncertain whether the slower component represents a hitherto unrecognized slowly turning over pool of bile acid or is a metabolic product of 23-selena-25-homotaurocholate not yet identified in vitro. There is a significant relationship between the whole-body retention of 23-selena-25-homotaurocholate and total fecal and primary bile acids. 23-selena-25-homotaurocholate is simple and acceptable for investigating ileal function.