The new marker CA 549 was determined in the serum of 258 breast cancer patients, classified according to TNM (148 at diagnosis and 110 at relapse), using a RIA method (cut-off: 10 U/ml). CEA, CA 15-3 and MCA were also evaluated. At diagnosis, CA 549 was more sensitive than the other markers, and cut-off values of 11 and 12 U/ml did not significantly reduce sensitivity. No significant correlation existed between the markers, except for CA 15-3 and CA 549 (r = 0.65). A new quantitative approach to the four markers was effected in the relapsed patients: an X value was calculated for each marker by dividing serum concentration by its cut-off. In these patients, grouped according to the area involved, marker sensitivities were similar except in locoregional relapse, where CA 549 and MCA were the most sensitive. From the data obtained, the more defined cut-off and the good specificity, it is suggested that CA 549 be routinely determined in the follow-up of the disease.