Abstract: An unusual diagnostic procedure for the in vivo clinical evaluation of gastrointestinal ulcer disease and other diseases associated with loss of mucosal integrity in both animals and humans which comprises the oral administration of an effective amount of radiolabeled colloidal bismuth subcitrate or other ulcer avid bismuth compound to an animal or human host, followed by scintigraphic imaging of the gastrointestinal area of interest with scintigraphic imaging equipment. Also disclosed is a novel radiopharmaceutical composition comprising an aqueous solution or suspension containing an amount of a radiolabeled bismuth-protein complex effective for in vivo scintigraphic imaging of the gastrointestinal or other mucosal areas in animals and humans.