Recent evidence suggests that mesenchymal stem cells (MSC) selectively home to tumors, where they contribute to the formation of tumor-associated stroma. This effect can be opposed by genetically modifying MSC to produce high levels of anti-cancer agents that blunt tumor growth kinetics and inhibit the growth of tumors in situ. In this review article, we describe the biological properties of MSC within the tumor microenvironment and discuss the potential use of MSC and other bone marrow-derived cell populations as delivery vehicles for antitumor proteins.