A quantitative study of carcinoembryonic antigen (CEA) was made in nine human gastric cancer cell lines. Six of them were found to produce CEA in vitro. The production of CEA in the three cell lines derived from well differentiated tubular adenocarcinomas began at the mid-exponential phase of cell growth and reached its peak at the late stationary phase, the amount of CEA per 10⁵ cells and the frequency of CEA-positive cells on immunostaining increased with culture time. In contrast, CEA in the three cell lines derived from poorly differentiated adenocarcinomas including a signet-ring cell carcinoma was produced immediately after plating and the amount of the antigen per 10⁵ cells and the frequency of CEA-positive cells were almost constant throughout the cell growth phases. Serum CEA content in nude mice was low or not detectable in the case of subcutaneous heterotransplantation of gastric cancer cells, irrespective of CEA productivity of the cell lines in vitro. Intraperitoneal inoculation, however, led to high CEA levels in sera of nude mice bearing human gastric cancers. No significant difference was found between the two kinds of inoculation in terms of the total tumor weight and the frequency of CEA-positive cells in tumor tissues. One reason for the above findings may be that the transport of CEA in the subcutaneous tumors to the systemic blood flow is hindered.