A monoclonal antibody (C 50) was recently produced by immunization with a colorectal cell line defining the tumour associated carbohydrate antigen CA-50. In previous studies CA-50 in serum has been elevated in 60-70% of patients with gastrointestinal carcinomas. This study surveys the serum levels of CA-50, as determined by a radioimmunoassay, in pancreatic disease. Elevated levels of CA-50 were found in 22 of the 24 patients with carcinoma of the exocrine pancreas, in one of eight patients with chronic pancreatitis, and in six of fourteen attacks (41%) of acute pancreatitis. The serum CA-50 antigen, as determined by the assay employed here, has a high sensitivity (92%) for exocrine pancreatic carcinoma. Only patients with malignant disease had CA-50 values exceeding 43 units per ml. In 58% of the patients with pancreatic carcinoma (14/24), CA-50 values surpassed 43 units per ml. In the presence of a pancreatic mass, marked elevation seems to support diagnosis of carcinoma. The low positive predictive value makes the assay unsuitable as a screening test for detection of pancreatic carcinoma in a general population.