Human CD8⁺ tumor-infiltrating T lymphocytes (TIL), in contrast with CD8⁺ blood cells, show impaired IFN-γ secretion on ex vivo restimulation. We have attributed the impaired IFN-γ secretion to a decreased mobility of T-cell receptors on trapping in a lattice of glycoproteins clustered by extracellular galectin-3. Indeed, we have previously shown that treatment with N-acetyllactosamine, a galectin ligand, restored this secretion. We strengthened this hypothesis here by showing that CD8⁺ TIL treated with an anti–galectin-3 antibody had an increased IFN-γ secretion. Moreover, we found that GCS-100, a polysaccharide in clinical development, detached galectin-3 from TIL and boosted cytotoxicity and secretion of different cytokines. Importantly, we observed that not only CD8⁺ TIL but also CD4⁺ TIL treated with GCS-100 secreted more IFN-γ on ex vivo restimulation. In tumor-bearing mice vaccinated with a tumor antigen, injections of GCS-100 led to tumor rejection in half of the mice, whereas all control mice died. In nonvaccinated mice, GCS-100 had no effect by itself. These results suggest that a combination of galectin-3 ligands and therapeutic vaccination may induce more tumor regressions in cancer patients than vaccination alone. Cancer Res; 70(19); 7476–88. ©2010 AACR.