The interaction of cells with the extracellular matrix regulates cell adhesion, motility, growth, survival and differentiation through integrinmediated signal transduction. Here we demonstrate that galectin-8, a secreted mammalian β-galactoside binding protein, inhibits adhesion of human carcinoma (1299) cells to plates coated with integrin ligands, and induces cell apoptosis. Pretreatment of the cells with Mn²⁺, which increases the affinity of integrins for their ligands, abolished the inhibitory effects of galectin-8. The inhibitory effects of galectin-8 were specific and were not mimicked by plant lectins or other galectins (galectin-1 and galectin-3). In accordance with its antiadhesive effects, transfection of galectin-8 cDNA into 1299 cells significantly reduced (by 75%) colony formation, when compared to the number of colonies formed by cells transfected with an empty vector. Affinity chromatography over immobilized galectin-8 indicated that few membrane proteins interacted with galectin-8 in a sugardependent manner. Microsequencing and western immunoblotting revealed that α₃β₁ integrin derived from 1299 as well as other cells (e.g. HeLa and human endothelial cells) is a major galectin-8 bindingprotein. Furthermore, immunoprecipitation and immunohistochemical studies suggested that endogenous galectin-8, secreted from 1299 cells, forms complexes with α₃β₁ integrins expressed on the surface of 1299 cells. Galectin-8 also interacts with other members of the integrin family, like α₆β₁ integrins. In contrast, galectin-8 only minimally interacts with α₄ or β₃ integrins. We propose that galectin-8 is an integrin bindingprotein that interacts to a different extent with several, but not all members of the integrin family. Binding of galectin-8 modulates integrin interactions with the extracellular matrix and thus regulates cell adhesion and cell survival.