The monoclonal antibodies CA15-3 were developed against the two antigens 115D8 of the human milk fat globule membrane and DF3 of breast cancer. CA15-3 was assayed radioimmunologically and CEA was analysed using the enzyme immunoassay. Normal control was achieved in 32 healthy women, the mean values for CA15-3 were 11.5 +/- 3.0 u/ml, range from 7.9 to 16.9 u/ml. We compared serum levels of CA15-3 and CEA in 121 patients with histologically proved breast carcinoma. CA15-3 levels above 25 u/ml and CEA levels above 5 ng/ml were considered positive values. 31 of 121 patients studied had elevated CA15-3 levels (sensitivity: 25.6%) and 21 of 121 patients had positive CEA levels (sensitivity 17.4%). 92 of the breast cancer patients (76%) did not have metastatic disease. In this group CA15-3 sensitivity was 7.6%, while CEA sensitivity was 6.5%. Mean values were 15.1 +/- 6.6 u/ml for CA15-3 and 1.78 +/- 2.47 ng/ml for CEA. 29 patients (24%) had metastatic disease. In this group, CA15-3 sensitivity was 82.8% and CEA sensitivity was 51.7% (P < 0.05). Mean values for CA15-3 were 147.5 +/- 175.9 u/ml and 16.9 +/- 24.0 ng/ml of CEA. With regard to the correlation of two tumor markers with clinical course patients had significantly higher levels of CA15-3 than of CEA in metastatic breast cancer. This result suggests CA15-3 to be the more sensitive and more specific of the two for metastatic breast cancer detection and monitoring.