A radio-opaque white powder used in X-ray examinations of the stomach and gastrointestinal tract. It may be swallowed to enable the oesophagus, stomach and small and large intestines to be assessed for disorders such as ulceration, tumours, DIVERTICULAR DISEASE and polyps. It may also be inserted into the RECTUM or descending COLON by enema to investigate for possible disease. These procedures are usually done after endoscopy examinations have been carried out.