MCA (mucinous-like cancer antigen) can be measured in the biological fluids of patients by means of a solid phase enzyme immunoassay. This study describes the results of MCA determination in sera of 230 patients with benign (99) and malignant (131) breast diseases. MCA levels were significantly higher in breast cancer patients than in non cancer patients and in healthy subjects (p less than 0.001). MCA concentrations tended to increase as the stage of the disease advanced. The 95th percentile of MCA value distribution in normal subjects showed a diagnostic sensitivity in breast cancer patients of 16.3% at stage I, 26.2% at stages II-III and 52% at stage IV. In a group of 118 cancer patients, MCA and CEA were tested simultaneously. The diagnostic sensitivity and specificity of MCA and CEA assays was very similar; nevertheless the association of the two tests showed 11 cases with high levels of MCA and low levels of CEA and 9 patients with high levels of CEA and low levels of MCA. Seventy-four out of 118 patients were negative for both markers and in 22 out of 118 patients markers were positive. The new marker MCA appeared to correlate with breast cancer and gave different information complementary to CEA.