Sandwich radioimmunometric assay (RIA) for a new tumor-associated carbohydrate antigen defined by a monoclonal antibody (MoAb), NCC-ST-439, was developed and the antigen levels were determined in sera of normal donors, and patients with various malignant and non-malignant disorders. In normal donors, 97.0% (226/233) of sera were antigen-negative (less than 12 units/ml) except for 7 serum samples from young females. In patients with malignant disorders, 34.2% (82/240) were antigen-positive, in particular 64.0% (16/25) of patients with pancreatic carcinoma, 66.7% (16/24) of patients with recurrent colorectal carcinoma and 54.5% (6/11) of patients with recurrent breast carcinoma. In patients with non-malignant disorders, 6.0% (7/116) were antigen-positive. The positive rate in benign hepatobiliary disorders, including gallstones, hepatitis and liver cirrhosis, was especially low at 4.3% (1/23). We concluded that determination of serum NCC-ST-439 antigen would be useful in serodiagnosis of carcinoma patients.