Carcinoembryonic antigen (CEA) is a cancer-specific antigen described by Gold in 1965. It is a glycoprotein present in malignant entodermal tissues, in fetal colonic mucosa, and in the plasma of patients with gastrointestinal tract cancers. We have measured plasma CEA levels in 346 patients using a new procedure which detects an ion-sensitive antigenic site in the carcinoembryonic molecule. Levels in 39 of 48 patients with gastrointestinal tract cancer and 90 of 281 with nonentodermally derived malignancies were abnormal. Following successful therapy CEA level returns to normal (colon, four of four; neuroblastoma, three of three). The identity between the antigen measured in the present study and the carcinoembryonic antigen of Gold has not been established. The assay appears to be useful in the follow-up of patients with cancer and may eventually be of diagnostic value in the asymptomatic patient.