 Tumor resistance to CTL killing is the most likely explanation for the poor effectiveness of cancer vaccines. This resistance is probably acquired by the tumor during its development and selected by its repetitive challenge with spontaneous anti-tumoral immune responses. Recently, we have identified a novel mechanism causing anergy of tumor-associated T lymphocytes, and established new approaches to correct this anergy in vitro. Galectin-3, secreted by tumor cells, appears to inhibit CTL function the co-localization of the T cell receptor and the cluster of differentiation 8 coreceptor. Importantly, this functional defect is restored when anergic T cells are incubated with galectin-3 inhibitors such as the disaccharides lactose and N-acetyllactosamine, and the polysaccharide GM-CT-01. GM-CT-01 is a vegetal galactomannan oligomer, currently under clinical investigation in several types of solid malignancies for its capacity to inhibit galectins and to synergize with chemotherapy drugs. 