T cells mature in the thymus, and then migrate to organs such as spleen and lymph nodes, where they participate in the immune response to foreign invaders. We have found that a carbohydrate binding protein, termed galectin-1, is expressed in numerous cell types in humans and mice, including organs of the immune system, immune privileged tissues, and tumors. Galectin-1 induces apoptosis, or programmed cell death, of T cells both during development in the thymus and after immune stimulation in the periphery, indicating that galectin-1 plays a role in shaping and regulating the immune response. Galectin-1 induced cell death occurs via a novel, caspase-independent pathway.