The levels and character of carcinoembryonic antigen (CEA) in feces were investigated by sandwich radioimmunometric assay using anti-CEA monoclonal antibodies NCC-CO-411 and NCC-CO-432. Mean CEA concentration was significantly higher (P less than 0.001) in the feces from patients with colorectal carcinoma and other gastrointestinal disorders as compared to normal adults. More than 90% of the fecal CEA was trapped by a 0.22 micron membrane filter and solubilized by treatment with 1% Triton X-100 or phosphatidyl-inositol specific phospholipase C. In hydrophobic chromatography, most of the fecal CEA was eluted at the lowest (NH4)2SO4 concentration while serum CEA appeared in the more hydrophilic fractions. These results suggest that the majority of CEA exists in feces as an amphiphilic molecule or a membrane-bound form. The increase of fecal CEA may reflect the destruction and abrasion of epithelial cells in various gastrointestinal disorders.