CA 72 is a monoclonal antibody (MAb) -defined antigenic determinant expressed on a pancarcinoma antigen (TAG-72) found in more than 85% of human colorectal carcinomas. An immunoradiometric assay has been developed using the murine MAb B72.3 to quantitate CA 72 in human serum. In a simultaneous immunoradiometric assay, the mean CA 72 concentration in 1,099 serum samples from healthy blood donors was 1.83 +/- 2.03 (SD) units/ml. If the upper limit of normal was set at 10 mu/mol of serum, a value including 99% of healthy blood donors, only 4 of 101 serum samples (4%) from patients with benign disease were elevated, whereas 15 of 26 (58%) and 14 of 25 (56%) of rectal and colon carcinoma patient sera, respectively, were positive. Serum samples from 84 benign colorectal disease cases were examined; of these, 0 of 28 (0%) colorectal adenoma, 1 of 39 (3%) ulcerative proctocolitis, 0 of 15 (0%) diverticulosis, and 0 of 2 (0%) irritable bowel disease sera contained more than 10 mu/ml CA 72. At a reference value of 20 mu/ml, 0 of 101 (0%) benign disease and 2 of 1,060 (0.2%) blood donor sera had elevated values, whereas 10 of 26 (38%) and 9 of 25 (36%) rectal and colon patient sera, respectively, remained positive. The majority of patients with pancreatic and ovarian cancer, and a significant fraction of stomach cancer patient sera, also contained elevated levels of CA 72. The ability of this assay to discriminate between malignant and benign diseases suggests its further evaluation for monitoring and diagnosis in groups at risk for development of cancer.