A competition radioimmunoassay was developed, utilizing a murine monoclonal antibody to human pancreatic adenocarcinoma cells. Immunoblotting of a standard antigen preparation from either serum or ascites fluid after electrophoresis in 1% agarose showed that the specific DU-PAN-2 activity resided in two major high molecular weight bands. DU-PAN-2 antigen levels were expressed as arbitrary units based on a standard partially purified antigen preparation. The inhibition curve with standard antigen was reproducible (SD less than 10%) and essentially linear from 25 to 200 units/ml. The mean DU-PAN-2 antigen concentration for the sera from 126 normal individuals was 81 units/ml. Sera from pediatric patients with malignancy had a mean of 127 units/ml, while nasopharyngeal, stage III melanoma, and ovarian carcinoma patients had means of 89, 92, and 119 units/ml, respectively. All values in normal subjects as well as the melanoma, nasopharyngeal, ovarian, and pediatric cancer patients were less than 400 units/ml. Intermediate antigen levels were detected in patients with alimentary tract malignancies. Eight of 20 gastric cancer and 8 of 76 colorectal carcinoma patients and 3 patients with benign or nonmalignant gastrointestinal tract disease had DU-PAN-2 values exceeding 400 units/ml. Ascites fluids from 6/6 and pancreatic juice from 2/2 pancreatic cancer patients had values greater than 750 units/ml. Serum from 68% of the 89 pancreatic cancer patients tested had DU-PAN-2 antigen levels greater than 400 units/ml. The mean serum value in this patient population was 4888 units/ml.