Inflammation resultant from tumor growth, infection with certain pathogens, or in some cases, trauma, can result in systemic release of cytokines, especially GM-CSF, that in turn stimulate the abundant production and activation of a population of immature myeloid cells, termed myeloid suppressor cells (MSCs), that have potent immunosuppressive functions. In this issue of the JCI, Gallina and colleagues have illuminated some complex issues concerning the development, activation, and function of MSCs (see the related article beginning on page 2777). They show that activation of MSCs is initiated in response to IFN-γ, presumably produced in situ by antitumor T cells in the tumor microenvironment. After this triggering event, MSCs express 2 enzymes involved in l-arginine metabolism, Arginase I and iNOS, whose metabolic products include diffusible and highly reactive peroxynitrites, the ultimate biochemical mediators of T cell immune suppression. The multifaceted regulation of this complex suppressive effector system provides several potential therapeutic targets.