DAB(389)IL-2 (denileukin diftitox, ONTAK) is an interleukin-2 receptor (IL-2R)-specific ligand fusion protein that may potentially be selective for IL-2R-expressing malignancies. The activity of DAB(389)IL-2 in the treatment of cutaneous T-cell lymphoma has established the feasibility of utilizing such a targeted therapeutic in disseminated disease with acceptable toxicity. Data from the phase I trial suggest that the definition of activity in other cancer types, including other non-Hodgkin's lymphomas (NHL), is warranted. Three NHL patients in this study responded, two of whom had follicular lymphomas, with the third having a primary intermediate-grade B-cell NHL that was refractory to chemotherapy and stem cell transplant. This patient has remained in complete remission over 3 years after treatment with DAB(389)IL-2. Patients treated to date have had IL-2R-positive tumors, but this remains a very complex clinical issue. The need for a threshold level of receptor expression, the difficulty in obtaining representative tissue, the lack of an assay that accurately reflects high-affinity receptor, and the potential difficulty of observer variability in evaluating the assays should point us toward examining response rates in cancer patients where IL-2R cannot be detected or is unknown. The potential to target the high-affinity IL-2R supports the development of this agent in transplantation and in autoimmune diseases. Targeting IL-2R-expressing lymphocytes may be an effective strategy for the prevention of graft rejection and to treat or prevent graft-versus-host disease. DAB(389)IL-2 has been examined in clinical trials of psoriasis and rheumatoid arthritis and has shown promising results. The potential utility in other autoimmune disorders is unknown, but diseases such as systemic lupus, scleroderma, and vasculitis also may be effective candidates for such ligand fusion therapy.