CA 549, a new marker for breast cancer, was measured in serum of 719 patients by an immunoradiometric assay involving two monoclonal antibodies: BC4E 549, developed against a breast-tumor cell line, and BC4N 154, developed against milk fat-globule membrane. The reference interval for healthy women was 0-11 kilo-units/L. The percentages of patients with CA 549 greater than 11 kilo-units/L for benign conditions are: 0% pregnancy, 1% breast, 26% liver; and for nonbreast metastatic cancers: 12% endometrial, 33% lung, 40% prostatic, and 50% ovarian. In women with breast cancer who were receiving or had completed adjuvant therapy with no evidence of disease there was an 11% increase in CA 549. For patients with metastatic breast cancer, 19% of those in complete remission, 63% of those in partial remission, and 88% of those with systemic progression had increased CA 549. CA 549 is a more specific marker than carcinoembryonic antigen (CEA) in nonmalignant disease, nonbreast malignancies, and adjuvant breast-cancer patients, and it is more sensitive in breast-cancer patients with progressive disease than is CEA. We could show CA 549 to be superior to CEA for detecting active breast cancer in patients with malignant or nonmalignant breast diseases. In monitoring 19 adjuvant-treated patients, CA 549 correlated more closely with the clinical course than did CEA values and, when increased, predicted a clinical recurrence. In 18 breast-cancer patients with metastasis, monitored for two to three years, the change of CA 549 values paralleled disease courses more often than did CEA values.