Serum concentrations of the CA 19-9 and CA 50 antigens were determined in 129 patients with malignant and benign biliary and pancreatic diseases. Values for the two markers were highly correlated (P less than 0.001). The concentrations of CA 19-9 and CA 50 were positive in 84.6% and 80.7% of patients with pancreatic cancer, respectively. The overall specificity of CA 19-9 (92.4%) was slightly higher than that of CA 50 (88.5%). The sensitivity of CA 50 (91.3%) was greater than that of CA 19-9 (73.9%) in patients with diseases of the biliary tract. Elevated concentrations of CA 19-9 (12.9%) and CA 50 (35.2%) were also found in a number of cases with benign disease, especially in patients with obstructive jaundice. These data suggest that both CA 19-9 and CA 50 can be useful markers of pancreatic cancer in nonjaundiced patients. The joint use of the two markers does not yield a better diagnostic resolution than the use of either one alone.