A wide variety of human adult and fetal tissues were studied by immune-diffusion techniques in agar gel to determine whether they contained the tumor-specific antigen(s) previously found in coionic cancers. In the adult tissues it was demonstrated that identical antigens were present in all tested specimens of malignant tumors of the entodermally derived epithelium of the gastrointestinal tract and pancreas, but were absent from all other tested adult tissues. The common antigenic constituents, therefore, represent system-specific cancer antigens of the human digestive system. System-specific cancer antigens have not previously been demonstrated in humans. Experiments with fetal tissues demonstrated that identical antigens were also present in fetal gut, liver, and pancreas between 2 and 6 months of gestation. These components were named "carcinoembryonic" antigens of the human digestive system. On the basis of the present findings and the recent work regarding control of the expression of genetic potentialities in various types of cells, it was concluded that the carcinoembryonic antigens represent cellular constituents which are repressed during the course of differentiation of the normal digestive system epithelium and reappear in the corresponding malignant cells by a process of derepressive-dedifferentiation.