An oncofetal pancreatic antigen (OPA) has been identified and purified from the blood of patients with pancreatic cancer. Characterisation studies on OPA have shown that it is a protein of molecular weight 40 000 with alpha2 electrophoretic mobility. OPA is clearly different from alpha-fetoprotein, carcinoembryonic antigen, ferritin, acute phase reactants, and normal serum proteins. A rocket immunoassay has been developed allowing the quantitation of OPA in serum; it has been applied to samples from over 700 individuals with a variety of conditions. Elevated levels of OPA have been found in 42 of 48 (88%) patients with biopsy-proven pancreatic cancer and in a much smaller percentage of patients with other cancers or with other conditions considered in the differential diagnosis of pancreatic cancer. The studies indicate that serum OPA measurements may be useful as a preliminary screening test for pancreatic cancer and for monitoring the course of the disease.