GALECTIN-1, a member of the family of beta-galactoside binding proteins(1), has growth regulatory and immunomodulatory activities(2-4). We report here that galectin-1, expressed by stromal cells in human thymus and lymph nodes(5,6), is present at sites of cell death by apoptosis during normal T-cell development and maturation, Galectin-1 induced apoptosis of activated human T cells and human T leukaemia cell lines. Resting T cells also bound galectin-1, but did not undergo apoptosis. Human endothelial cells that expressed galectin-1 induced apoptosis of bound T cells. Galectin-1-induced apoptosis required expression of CD45, and was decreased when N-glycan elongation was blocked by treatment of the cells by swainsonine(7), whereas inhibition of O-glycan elongation(8) potentiated the apoptotic effect of galectin-1. Induction of apoptosis by an endogenous mammalian lectin represents a new mechanism for regulating the immune response.