Serum CA M26 and CA M29 values were determined in 125 women: 46 patients with primary breast cancer, 49 patients with benign breast disease, and 30 controls. The mean CA M26 and CA M29 values in breast cancer patients were higher than in patients with benign breast disease and in control subjects. When we set the cut-off level at 40 kU/1 for CA M26, 15/46 (33%) of all breast cancer patients, 11/20 (55%) of breast cancer patients with axillary node involvement, 1/1 breast cancer patient with distant metastases, 4/49 (8%) of patients with benign breast disease, and 7/30 (23%) of control subjects were above this cut-off level. For comparison, at the cut-off level of 10 kU/1 the CA M29 test was positive in 30/46 (65%) cancer cases, in 16/20 (80%) of cancer patients with nodal involvement, in 9/49 (18%) of the patients with benign breast disease, and in 10/30 (33%) of control subjects. Patients with axillary nodal metastasis showed higher values than patients without metastasis in both tests. When we used a cut-off level of 15 kU/1 for CA M29, 24% (11/46) of all breast cancer patients, 35% (7/20) of breast cancer patients with axillary node involvement, 4% (2/49) of patients with benign breast disease, and 0% of control subjects were above this cut-off level. The combination of CA M26 at the 40 kU/l cut-off level and CA M29 at the 10 kU/l cut-off level reached the diagnostic sensitivity of 0.65, specificity of 0.78, and efficiency of 0.72. In breast cancer detection among patients with breast problems serum marker CA M29 reached the sensitivity of 0.65 at the specificity level of 0.82. Our results suggest that the CA M29 marker alone is as good as the panel of two markers (CA M26, CA M29) in breast cancer diagnostics. Thus it seems that CA M29 is a promising serum tumor marker in breast cancer.