The expression of carcinoembryonic antigen (CEA) by tumor cells from freshly excised human gastric cancers was investigated using flow cytometry (FCM). Highly purified fresh human cancer cells were obtained from solid tumors in 20 patients and from malignant ascites in 6 patients. Thirteen of the 26 tumors were positive for CEA by FCM. CEA expression was more common in well-differentiated tumors than in poorly differentiated tumors. CEA expression was investigated by both FCM and immunohistochemistry in 9 patients, and the two methods agreed in 8 of them. However, quantitative evaluation of CEA expression could only be performed by FCM and not by immunohistochemical staining. FCM could analyze the expression of CEA not only on the cell membrane but also in the cytoplasm, by using gastric cancer cells with or without Triton X-100 treatment. Thus, this study showed that CEA expression can be determined and evaluated quantitatively by FCM.