Abstract: Serum CA50 antigen in the nomal individuals and carcinoma of the stomach was assayed by radioimmunoassay,CA50,one type of carbohydrate antigen,consisting mainly of sialylated glycolipid(a ganglioside)and sialylated,Can be isolated from the primary or metaslatic tumors.The serum CA50 content was measured in 52 nomal individuals,62 patients with carcinoma of the stomach and 32 pstients with benign disease of the stomach.The normal value was 3.80±2.45ku/L.The test values in cancer patients were significantly higher than those in normals or benign disease patients.Taking the normal mean value plus two times of standard deviation as positive,the positive rate was 82.3% for Carcinomar of stomach.There was only two positive in 52 normals with a false positive rate of 3. 7%. Furthermore, 38 patients,Cured by effective treatment,gave near normal CA50 value,It is shown that this metsod can be used in carcinoma of stomach diagnosis,differential diagnosis and the monitor, prediction of prognosis.