Tumour-associated antigen CA 19-9 was determined in serum from 166 patients (30 without gastro-intestinal disease, 32 with liver cirrhosis, 9 with choledocholithiasis, 65 with acute or chronic pancreatitis and 30 with malignant tumors in the region of pancreas and bile passages). The specificity of CA-19-9 as tumour marker was 97% in patients without gastro-intestinal disease, but in those with liver cirrhosis or choledocholithiasis it was only 56% and 44%, respectively. In particular, cholestasis reduced specificity. Acute pancreatitis in its initial attack gave false-positive CA 19-9 values in 27% of cases, repeated bouts in chronic recurrent pancreatitis in as many as 50%. In chronic pancreatitis the specificity was 90%. Malignant tumours of pancreas and bile ducts were diagnosed with a sensitivity of 80%. Determination of CA 19-9 in pure pancreatic secretion failed to differentiate between the control group (30), chronic pancreatitis (21) and carcinoma of the pancreas (22).