Side effects associated with current immunosuppressive therapy complicate the use of islet transplantation as a treatment for type 1 diabetes. Multipotent mesenchymal stromal cells (MSCs) have demonstrated immunomodulatory activity. The purpose of this study was to investigate whether a murine stromal cell line affects graft rejection in a fully major histocompatibility complex (MHC)-mismatched islet transplant model. We show that stromal cells have an inhibitory effect on T cell proliferation in vitro, and that they slow down the rejection of allogeneic islets. These findings indicate a possibility to use MSCs as a treatment to prolong the survival of islet grafts.