We established a human gastric cancer xenograft which, when inoculated into nude mice, showed a positive correlation between tumor growth and the serum level of carcinoembryonic antigen (CEA). Serum CEA levels in the mice rose continuously with increasing tumor weight after inoculation, showing a correlation coefficient of 0.96. A positive correlation was also observed between the tissue CEA level and tumor weight, the former increasing along with the latter. Furthermore, the level of serum CEA closely paralleled that of tissue CEA. The serum CEA level fell after tumor extirpation, with a half-life of approximately 86 h. These results suggest that the elevation of serum CEA is attributable to the gain in tumor weight as well as the increase of CEA production in the tumor tissue. Thus, human gastric cancer xenografts in nude mice are a good model for examining the biological role of CEA.