The Wiskott-Aldrich syndrome (WAS) interacting protein (WIP) stabilizes the WAS protein (WASP), the product of the gene mutated in WAS. WIP-deficient T cells have low WASP levels, limiting the usefulness of WIP KO mice in defining the role of WIP in T cell function. To define this role, we compared WIP/WASP double KO (DKO) mice to WASP KO mice on DO11.10 background. T cell development was normal in both strains, but peripheral T cell numbers were significantly decreased in DKO mice. WASP KO T cells proliferated and secreted IL-2 normally in response to OVA peptide (OVAp). In contrast, T cells from DKO mice proliferated poorly in response to OVAp in vitro, and cutaneous hapten hypersensitivity was deficient in these mice. DKO T cells up-regulated CD25 expression and secreted normal amounts of IL-2 after antigen stimulation, but had defective response to IL-2, evidenced by failure to further up-regulate CD25 expression, phosphorylate STAT5, and induce expression of STAT5-dependent genes. DKO, but not WASP KO, T cells had a disrupted subcortical actin cytoskeleton and impaired actin polymerization after T cell antigen receptor (TCR) ligation. These results indicate that WIP is essential for IL-2 signaling and responsiveness in T cells, possibly because of its critical role in TCR-triggered actin cytoskeletal reorganization.