It is used for quantitative detection of carcinoembryonic antigen (CEA) in human serum in vitro. Carcinoembryonic antigen (CEA) is composed of some glycoproteins with molecular weight of 175-200kd. These glycoproteins vary with the amount of carbohydrate and amino acid. Although the biological function of CEA is not well defined, it is still the most widely used marker of colorectal cancer. In addition to colorectal cancer, other malignant tumors such as breast cancer, lung cancer, gastric cancer, pancreatic cancer, ovarian cancer and so on will cause the increase of CEA level. Benign conditions, including pneumonia, gastrointestinal tract inflammation, benign liver tumor and so on, also make CEA level higher than normal. The CEA content in serum of healthy adults is less than or equal to 5.0ng/ml. When the CEA content is more than 5 times of the normal value, it is considered as the manifestation of malignant tumor. Because of the overlapping of malignant and non malignant conditions, only CEA can not be used as a criterion for malignant tumors.