The clinical utility of CA M26 and CA M29 was studied in 116 breast cancer patients and compared with results for CA 15-3 and carcinoembryonic antigen (CEA). The highest sensitivities for breast cancer detection were achieved with CA 15-3 (0.60) and CEA (0.56), but this was compromised by a relative lack of specificity (0.87 and 0.88 for CA 15-3 and CEA, respectively). Sensitivities attained with CA M26 (0.47) and CA M29 (0.53) were lower, but there was an excellent specificity (1.00) for each assay in this series of benign patients. Tumor marker elevations were appreciable with advanced disease such that 82 of 91 patients (90%) with active metastatic breast cancer exhibited at least one abnormal test value. Longitudinal studies demonstrated that CA M26, CA M29, CA 15-3 and CEA complement each other and combinations of these markers reflect disease status better than individual tests.