The monoclonal antibody-defined CAR-3 antigen is a new carcinoma associated marker which is expressed on a mucin-like molecule. Serum concentrations of CAR-3 were assayed in 181 patients with carcinomas of different organs, 20 patients with non-carcinomatous malignancies, 123 patients with inflammatory diseases and 150 healthy controls. Serum levels of CAR-3 were significantly increased in 51% of the patients with pancreatic carcinomas, in 60% of patients with biliary tract carcinomas and in about 15% of the patients with carcinomas of the digestive apparatus. Sera from patients with breast carcinomas were negative, as well as sera from patients with melanomas or sarcomas. CAR-3 values in samples from patients with chronic pancreatitis were constantly negative, as were samples from healthy donors. Significant concentrations of CAR-3 were detected in 20% of the sera from patients with acute pancreatitis and in 15% of the sera from patients with cirrhosis. Because of its high specificity for pancreatic carcinomas compared to chronic pancreatitis, CAR-3 seems a promising marker for distinguishing between neoplastic and chronic inflammatory diseases of the pancreas, whose differential diagnosis is difficult.