Although IL-2 is commonly thought to promote proliferation of T lymphocytes, mice deficient in IL-2 exhibit splenomegaly, lymphocytosis, and autoimmunity, suggesting this cytokine may have a prominent role in T cell homeostasis. Since the number of T cells in the bloodstream and lymphoid organs is tightly controlled, it is likely that the availability of IL-2 must also be closely regulated. One mechanism altering the local availability of cytokines is association with heparan sulfate, a glycosaminoglycan found on cell surfaces and within extracellular matrices. Here we show that an association between IL-2 and heparan sulfate localizes IL-2 to lymphoid organs such as the spleen. We also show that IL-2, sequestered in this way, contributes to the activation of T lymphocytes and primes T lymphocytes for activation-induced cell death.