Pancreatic oncofetal antigen (POA) is generally accepted as a useful tumor marker for the detection of cancer of the pancreas. However, reports on the biochemical characterization of POA have been contradictory. In the present study, biochemical and immunohistological characterizations of POA were performed with anti-fetal pancreas antibody specific to POA. POA of the fetal pancreas was located in the region of α(2)~β on immunoelectrophoretograms, and the molecular weight was estimated to be about 200,000 by gel filtration. POA did not react with anti-CEA, AFP, or ferritin antibodies in the Ouchtalony test. POA was also detected in extracts of cancerous pancreas by Ouchtalony test. From immunohistological studies with anti-POA serum, it was confirmed that POA was present in the cytoplasm of fetal and cancerous pancreas. Well differentiated tumor cells stained much more than poorly differentiated tumor cells. In conclusion, cancerous pancreas shares the same antigen, POA, with fetal pancreas, and the production of POA in cancerous pancreas is related to the degree of tumor cell differentiation.