CA 15-3, MCA, and CA-549 levels were determined in the serum of 56 patients with metastatic breast cancer, all of whom had visceral and/or bone metastases. CA 15-3 was positive in the serum of 37 patients, MCA in the serum of 38 patients, and CA-549 in the serum of 39 patients. All 3 markers were positive in 36 patients, and all 3 were negative in 16 patients. In the serum of 4 patients only 1 or 2 markers were slightly elevated. Thus, the results were identical in 52 of the 56 patients. It is concluded that CA 15-3, MCA, and CA-549 are sensitive markers for advanced breast cancer, and that no advantage can be expected by combining them.