The value of I131-labeled triolein in the demonstration of impaired intestinal absorption in diseases of the small bowel and of the pancreas and also following subtotal resection for peptic ulcer has been well established. The distinction between a defect of digestion and an error of absorption can be made by the successive use of I131-labeled triolein and the I131-labeled oleic acid. Co60-labeled B12, in conjunction with intrinsic factor and antibiotics, has been found valuable in distinguishing between sprue, pernicious anemia, and the blind-loop syndrome. Red blood cells tagged with Cr51have been used to demonstrate intestinal bleeding; I131-labeled albumin is helpful in determining plasma volume.