IL-2, a cytokine produced by T cells, is a key regulator of immune responses and T cell homeostasis. Controlling the availability of IL-2 is consequently of significant import to the immune system. Like other cytokines, IL-2 is thought to function as a soluble agonist, transiently present when secreted in response to appropriate stimuli. In this study, we show that the most salient properties of IL-2, propagation and control of T cell responses, are mediated in vivo by bound and not free cytokine and specifically by heparan sulfate-bound IL-2. These findings necessitate a new look at how IL-2 regulates immune responses and support the notion that the microenvironment plays a determining role in modulating the character of immune responses.