The serum carbohydrate antigenic determinant (CA 19-9) was assayed in patients with various diseases (87 patients with pancreatic carcinoma, 747 patients with benign diseases, and 547 patients with extrapancreatic malignant growths) and it proved to be particularly sensitive for adenocarcinoma of the pancreas (80 of 87, 92%) as compared to only 14% in the group of patients with benign diseases. Twenty-seven percent of the patients with chronic pancreatitis and 28% of the patients with acute pancreatitis showed elevated CA 19-9 concentrations of more than the upper normal value of 37 U/ml. In 38% and 32% of our cases with carcinoma of the stomach and colorectal carcinoma, respectively, CA 19-9 was estimated as being above the normal range. The preoperatively raised CA 19-9 concentration in patients with pancreatic carcinoma decreases after curative resection of the carcinoma to values within the normal range. However, in no CA 19-9 estimation following a palliative surgical intervention or in cases of inoperable carcinomas a serum concentration of less than 37 U/ml was recorded. In immunohistochemical specimens we found a difference between CA 19-9 antigen concentrations on the cell surface and secretion in pancreatic carcinoma and chronic pancreatitis.