The traditional chemical detection of small amount of blood in the stool can not tell whether the original bleeding site is in the upper or lower part of the G-I tract. The occult blood detector, designed by the authors, having a diameter of 8 mm, can easy be swallowed into the stomach with 30 to 50 ml of water. After remaining for 3 minutes in the stomach, it is pulled out by the patient himself and its coloration read to reveal the degree of bleeding. It can be used either by the medical personnel or the patient himself, at home, for the purpose of detecting minute bleeding in the upper G-I tract and for cancer screening in the high risk population. This detector gave a positive rate of 95% in patients with malignancies in the upper gastrointestinal tract. It is highly acceptable for its lack of discomfort during application. In cancer screening, it gave a positive rate of 5% to 10% in 10000 high risk persons. Those positive in this test showed a high incidence of gastric or esophageal cancers as proved by gastroscopy. This detector is being manufactured and sold by the Baiyunshan Pharmaceutical Factory, Guangzhou, Guangdong Province. Other papers concerning this topic are to be published.