A monoclonal antibody (3E1–2) based serum test using an enzyme immunoassay has been used to determine circulating levels of the breast cancer associated antigen—mammary serum antigen (MSA). Of 157 patients with early breast cancer (stage I and II) and 199 patients with advanced breast cancer (stage III and IV), 73 per cent and 87 per cent respectively had elevated MSA levels (i.e. > 300 inhibition units (IU). Furthermore, 40 of 44 patients (91 per cent) had a significant fall of MSA levels with reduction in tumour load by mastectomy. In addition, there was a correlation of MSA levels with the clinical course: changes in MSA levels correlated with changes in disease status (progressive disease, stable disease, disease regression) in 54 of 61 patients and antedated disease progression or recurrence by up to 8 months in some patients; and in 32 of 36 patients (89 per cent) with no clinical evidence of recurrence MSA levels did not vary by more than 25 per cent of the original MSA value over a period of 2–15 months. MSA is therefore a useful tumour marker in the diagnosis and staging of breast cancer. There is also evidence that serial estimations of MSA levels may be used to detect subclinical recurrence and the fluctuations in MSA levels might be useful in assessing response to therapy. Furthermore, it was also noted that surgical procedures such as fine needle aspiration biopsy or incisional biopsy could lead to a rise in MSA levels.