The galectins are a family of animal lectins that possess similar carbohydrate binding specificities and conserved consensus sequences. The biological properties of mammalian galectins include the regulation of inflammation, cell adhesion, cell proliferation and cell death. Evidence suggests that the biological activities of the galectins are related to their multivalent binding properties since most galectins possess two carbohydrate recognition domains and are therefore bivalent. For example, galectin-1, which is dimeric, binds and cross-links specific glycoprotein counter-receptors on the surface of human T-cells leading to apoptosis [J. Immunol. 163 (1999) 3801]. Different galectin-1 counter-receptors associated with specific phosphatase or kinase activities formed separate clusters on the surface of the cells as a result of the lectin binding to the carbohydrate chains of the respective glycoproteins. Importantly, monovalent galectin-1 is inactive in this system. This indicates that the separation and organization of signaling molecules that result from galectin-1 binding is involved in the apoptotic signal. The separation of specific glycoprotein receptors induced by galectin-1 binding was modeled on the basis of molecular and structural studies of the binding of lectins to multivalent carbohydrates resulting in the formation of specific two- and three-dimensional cross-linked lattices [Biochemistry 36 (1997) 15073]. In this article, the binding and cross-linking properties of galectin-1 and other lectins are reviewed as a model for the biological signal transduction properties of the galectin family of animal lectins.