Mucinous-like cancer antigen (MCA) is a substance which is recognized by a novel monoclonal antibody, designated b12, when directed against a mixture of human breast cancer cell lines. The present study was carried out by means of a solid phase enzyme immuno assay to determine, prior to treatment, circulating levels of MCA in 131 women with malignant and in 99 with benign breast diseases (BBD). Statistical analysis did not reveal any significant difference between MCA values in healthy controls vs. BBD patients. By contrast, MCA levels were significantly higher in cancer patients as compared to the other groups (p less than 0.001). In particular, patients with metastatic disease had the highest MCA concentrations. The sensitivity and specificity of the test were 25.5 and 97.5%, respectively. The characteristics of MCA (low sensitivity, high specificity) encourage its evaluation as an indicator of tumor burden.