Galectins (S-type lectins) are a family of low-molecular weight, calcium-independent, mannose-binding lectins with functions in cell growth, cell activation, cell-cell and cell-matrix adhesion including binding to carcinoembryonic antigens and laminin and metalloproteinase. Anti-galectin antisera can inhibit metastases of rat prostate cancers and human melanomas. To define the role of galectins in human breast cancer, the expression of galectin-3 were determined in 27 invasive breast cancers by immunohistochemical methods. The histologic grades of excised breast cancers were determined and immunohistochemical staining for galectin-3 (1: 1000 dilution of anti-galectin rat polyclonal antibody) was defined by scoring the intensity and distribution of staining (0-3+). The mean age of breast cancer patients was 63 years for 20 grade II breast cancers and 56 years for 7 grade III breast cancers. The mean immunohistochemical staining score for grade II breast cancers was 3. 7 (20% less than 2, 80% 3-6) and 2.5 for grade III (71.4% less than 2 and 28.6% 3-6). The galectin-3 expression pattern suggests that increasing histologic grade of breast cancer leads to reduced expression of galectin-3 and possibly reduced matrix binding and increased cancer cell motility.