Daclizumab is a humanized anti-interleukin-2 receptor monoclonal antibody that works by targeting and impairing activated T lymphocytes, a subset of white blood cells that has been thought to be involved in the development and maintenance of ITP. Daclizumab is a well-tolerated and time-limited therapy, and is easily administered on an outpatient basis. The purpose of this study is to test the efficacy of daclizumab as either a sole agent in the treatment of chronic, symptomatic ITP, or as a treatment that might allow a decrease or discontinuation of medications such as prednisone.