Human galectins are involved in a variety of biological and pathological processes including cell adhesion, apoptosis, differentiation, immune regulation and tumour evasion. Previously, we identified galectin-3 as the first human lectin involved in the modulation of the immunosuppressive potential of mesenchymal stem cells (MSCs). In this study, we report on the expression profiles and potential activities of other galectins expressed in these cells. The data show that MSCs constitutively express galectins-1, -3 and -8 at both the mRNA and protein levels. In contrast to galectin-8, galectins-1 and -3 are secreted and found on the cell surface. MSC-mediated T cell suppression was inhibited by galectin-1-specific siRNAs but not by galectin-8-specific siRNAs. The double knockdown of galectins-1 and -3 almost abolished the immunosuppressive capacity of MSCs. The use of a competitive inhibitor for galectin binding, ß lactose, restored alloresponsiveness, implying an extracellular mechanism of action of galectins. Collectively, the data highlight the involvement of secreted galectins-1 and -3 in MSC-mediated T cell suppression. The immunosuppression by MSC-secreted galectins should facilitate the use of recombinant galectin-1 and/or -3 as a novel therapy to alleviate inflammatory reactions such as those seen in graft versus host disease (GvHD) and autoimmune disorders.