The purpose of this study was to investigate the biologic activity of DAB486IL-2 when administered three times daily, in terms of toxicity, pharmacokinetics, and anti-tumor effects in patients with IL-2R expressing hematologic malignancies, especially mycosis fungoides. 20 patients were enrolled in this dose escalation phase I trial. Patient cohorts were treated at levels of 0.03 mg/kg, 0.05 mg/kg, 0.07 mg/kg and 0.09 mg/kg every 8 hours for a total of 12 doses, every 21 days as toxicity and response warranted. Eight patients experienced transient fevers associated with DAB486IL-2 administration. One patient experienced grade 3 hypotension, and a second developed fluid retention manifested as weight gain/pedal edema. Dose limiting toxicity consisted of one episode of transient grade 4 hepatic transaminase elevation, and 8 episodes of transient asymptomatic grade 3 hepatic transaminase elevation. At the maximum tolerated dose DAB486IL-2 exhibited biphasic clearance kinetics; transient receptor saturation may be one mechanism for this observation. Initial serum concentration and apparent steady state level (plateau) directly correlated with the dose administered, but no difference in area under the concentration curve with greater dose was observed. Biologic activity was noted in patients with mycosis fungoides with skin lesion clearing and relief of pruritus. One patient with mycosis fungoides, and one patient with a relapsed intermediate grade non-Hodgkin's lymphoma achieved partial responses. The novel mechanism of action, toxicity profile, and evidence of biologic activity in refractory cancer patients, support development of more active constructs of this agent; such trials are underway.