To establish a procedure for screening of pancreatic cancer, we assayed the serum level of immunosuppressive acidic protein (IAP) in patients with malignant and benign pancreatic diseases and compared it with the serum level of carcinoembryonic antigen (CEA). In 96% of 46 pancreatic cancer patients, the serum IAP level averaged 928 micrograms/ml; while 500 micrograms/ml or below was obtained as the normal value in 211 healthy adults. The serum IAP level may be elevated in the presence of non-malignant, inflammatory pancreatic diseases. For the differential diagnosis of pancreatic cancer, therefore, other laboratory data need to be taken into account. The serum IAP level was correlated with the progression of pancreatic cancer and may provide a means to determine whether tumor resection was radical or admitting of recurrence. The serum IAP level in samples obtained from different sites of the portal system did not reflect tumor location in patients with pancreatic cancer.