A comparative study of carcinoembryonic antigen (CEA) and beta 2-microglobulin in serum was made by radioimmunoassay in 121 patients with gastric disease. There was no statistical difference in serum CEA and beta 2-microglobulin in patients with benign gastric disease, early gastric carcinoma, and advanced gastric carcinoma. However, CEA levels in patients with advanced gastric carcinoma were higher than those in benign gastric disease and in early gastric carcinoma but beta 2-microglobulin did not show and differences. Furthermore, after total removal of the tumor, the elevated CEA levels decreased but beta 2-microglobulin remained unchanged. We concluded that CEA appeared to predict tumor behavior more accurately in patients with gastric carcinoma than beta 2-microglobulin.