Abstract: A new gastric cancer associated antigen MGdl-Ag were determinea in sera on 385 patients with tumors or benign diseases of the digestive system and normal health. The percentage of result was fouud tc be 14.3% in esophageal cancer, 18.6% in rectal cancer, 45% in gastric cancer, 50% in colonic cancer, 18. 7 in benign diseases and 2. 4% in normal health. It is seggusted that detection of MGdl-Ag in serum of patients may afford some aid in the diagnosis of gastric cancer and colonic cancer.