The histological growth pattern of large bowel carcinomas is probably of importance for CEA secretion, since it is demonstrated that moderately differentiated carcinomas have the highest plasma CEA levels. Moderately differentiated carcinomas might thus, because of their glandular architecture, secrete a larger proportion of CEA into the gut lumen than poorly differentiated ones. Nevertheless, of the biological variables investigated in relation to plasma CEA, determination of tumour DNA ploidy has turned out to be of the greatest significance in our laboratory. We suggest therefore, that combined evaluation of the preoperative plasma CEA level and tumour DNA ploidy may aid in the selection of patients who should be followed up with repeated plasma CEA measurements after potentially curative large bowel resection.