Carcinoembryonic antigen (CEA) levels in the feces and serum were evaluated in 22 colorectal cancer patients and 20 healthy volunteers; in CEA levels gastric juice and serum were also evaluated in 28 gastric cancer patients and 14 peptic ulcer patients. Fecal CEA was found in all of 22 colorectal cancer patients as well as in the 20 healthy volunteers. Elevated fecal CEA levels were observed in the colorectal cancer patients, as compared to the healthy volunteers. The feces of 15 of the 22 colorectal cancer patients contained CEA at concentrations higher than the mean value plus twice the standard deviation of the healthy volunteers. The fecal CEA levels did not correlate directly either with Dukes' stage or serum CEA levels. CEA in gastric juice was elevated significantly in 26 of gastric cancer patients, with the exception of two patients with early gastric cancer. On the other hand, serum CEA was elevated in only nine of the 28 gastric cancer patients. These results point out the distinct value of assaying CEA in the feces or gastric juice as an aid in the diagnosis of colorectal or gastric cancer.