Tumour-associated antigens CA 50 and CA 19-9 were determined in serum of 208 patients. Specificity of both neo-antigens as tumour markers was equally good, at 100% and 95%, in patients without malignancy or gastrointestinal disease, respectively, using an upper limit of normal of 17 U/ml for CA 50 and 37 U/ml for CA 19-9. Benign diseases of the upper gastrointestinal tract, such as pancreatitis, cholestasis or cirrhosis of the liver, reduce the specificity of CA 50 more than of CA 19-9. For example, specificity of CA 50 is only 33% for choledocholithiasis, but 74% with CA 19-9. The sensitivity of both closely related sialogangliosides in malignancies of the upper GI tract is similar, with the usual normal limits: in pancreas carcinoma 77% for CA 50, 81% for CA 19-9; in biliary tract carcinoma 80% for CA 50, 90% for CA 19-9; in gastric carcinoma 40% for CA 50, 50% for CA 19-9. But if one equalizes the upper limits of normal for both markers to a common 95% specificity, the tumour-indicating sensitivity of CA 19-9 clearly surpasses that of CA 50. Malignant tumours not recognized by increased levels of CA 19-9 also escape serological diagnosis with CA 50.