Murine monoclonal antibody 3E1.2, made against human breast cancer cells, detects a glycoprotein (Mr greater than 300,000) called mammary serum antigen (MSA) which is elevated in the serum of patients with breast cancer. An enzyme immunoassay was developed to detect MSA in human serum and used to detect MSA in subjects with breast cancer and other diseases. Raised levels of MSA (greater than 300 inhibition units) were found in the serum of 1.9% of 2406 blood donors, in 18% of sera from 40 subjects with benign breast disease, and in 16% of sera from 222 subjects with non-breast cancers. However, in patients with a diagnosis of breast cancer, 76% (84 of 110) of Stage I and II, and 86% (142 of 166) of Stage III and IV had levels of greater than 300 inhibition units. Nineteen % of patients, classified clinically disease free, had raised MSA levels. In 34 of 37 (92%) patients followed over 2 to 11 mo the level of MSA correlated with the clinical course of disease. Changes in MSA levels not only corresponded to changes in the clinical course of disease, but also preceded the clinical detection of progressive disease. Immunoblotting has detected a heterogeneous molecule of Mr greater than 300,000 and been used to confirm the elevation of MSA in breast cancer patients. Determination of MSA level may be useful for the detection of breast cancer and for monitoring progress of disease and response to therapy.