A study was made of the spread of a tumor process in 161 patients with gastric cancer and in 9 patients with colorectal cancer. Diagnostic examinations preceeded treatment. The concentrations of the carcinoembryonic antigen (CEA) and ..cap alpha..- fetoprotein (AFP) in the blood plasma were demonstrated in 170 patients. Liver scanning was done to 122, and computerized tomography to 18 patients. It has been shown that the determination of the CEA level in the blood plasma can serve as an additional parameter to define tumor spread in the gastrointestinal tract. In gastric and colorectal cancer, a hepatic large focal lesion is accompanied by the highest CEA level. The CEA level and the gastric primary tumor mass show direct correlation. No correlation has been found between the AFP level and the degree of spread of a tumor process, both local and metastatic.