Acute allograft rejection is driven by production of cytokines such as interleukin-2 (IL-2) that activate and expand alloreactive T cells by ligating high-affinity IL-2 receptors composed of three subunit chains: alpha, beta, gamma The alpha chain, expressed only on activated T cells, has become an important therapeutic target. Monoclonal antibodies (mAbs) that bind IL-2Ralpha chains significantly decrease transplant rejection. We examined the ability of the humanized anti-IL-2Ralpha antibody daclizumab to block high-affinity IL-2Rs and interrupt T-lymphocyte signaling. Our evaluation focused on a pathway critical for T-cell proliferation, the Jak/STAT pathway. Daclizumab markedly inhibited phosphorylation of the Jak1, Jak3 and STAT5a/b components of the IL-2R-dependent pathway. Suppression by daclizumab was associated with internalization of IL-2Ralpha but not IL-2Rbetagamma chains. High IL-2 doses overcame daclizumab-induced blockade of Jak/STAT phosphorylation despite absent cell surface highaffinity IL-2Rs. Under these circumstances, IL-2-mediated Jak/STAT pathway activation might be generated through residual intermediate affinity IL-2Rbetagamma receptors, and this was demonstrated by complete blockade of signaling when anti-IL-2Rbeta monoclonal antibody was added. Humanized antibodies are an important part of strategies to induce alloantigen tolerance. Understanding the molecular events associated with their beneficial clinical effect is critical to design of future immunosuppressive strategies.