A new tumour marker, CAR-3, was isolated using the monoclonal antibody technique and measured in the sera of 27 patients with pancreatic cancer, 25 chronic pancreatitis, 30 extra-pancreatic diseases and in that of 18 healthy controls in order (1) to evaluate the diagnostic role of CAR-3 in patients with pancreatic cancer and (2) to ascertain whether liver dysfunction influences CAR-3 serum levels. The increased levels were found in 12/27 patients with pancreatic cancer (sensitivity 44.4%). No increase was found in patients with chronic pancreatitis, whereas abnormal levels were found in patients with other gastrointestinal diseases, especially those of the liver and biliary tract. Correlations were found between serum CAR-3 and (1) total bilirubin and (2) alkaline phosphatase. In conclusion, CAR-3, an antigen structurally related to CA 19–9, does not appear to be accurate enough to be considered a tumour marker. Cholestasis seems to increase CAR-3 levels as well as those of other glycoproteic tumour markers, probably by interfering with the hepatic clearance of these substances.