Serum protein-bound carbohydrates, L-fucose, sialic acid, D-galactose, and D-mannose, were measured as potential biologic markers in patients with breast cancer with the use of high-resolution anion exchange separation in combination with a sensitive cerate oxidimetric fluorescence detector system. For 22 randomly selected patients with proved metastatic breast cancer, both L-fucose and sialic acid levels were above the normal range in 21 patients (95%). In contrast, D-mannose was increased in the sera of 9 patients (41%), and D-galactose in 6 (27%). By comparison, the level of carcinoembryonic antigen (CEA) was elevated in 14 patients (64%). The levels for serially determined serum protein-bound carbohydrates paralleled objective changes of response or progression in 5 patients with measurable disease. As might be expected, the degree and pattern of elevation for the individual carbohydrates varied for each patient studied. Combinations of serum protein-bound carbohydrates, particularly L-fucose and sialic acid, and, in addition, CEA, appear to have promise as potential biomarkers for following the course of the disease in patients with metastatic breast cancer.