Twelve patients were treated in a Phase I trial of purified human interleukin-2 (IL-2) derived from the JURKAT cell line (E.I. duPont Corp., Glenolden, PA, U.S.A.). The serum half-life, toxicity, and in vivo immunologic effects of IL-2 were studied in patients with cancer unresponsive to standard therapy and in patients with acquired immunodeficiency syndrome (AIDS). Patients received 0.25, 2.5, or 25 micrograms/kg IL-2 by bolus or 24-h continuous infusion on a weekly basis for 4 weeks. The serum half-life of JURKAT IL-2 in humans was approximately 6 min. At higher doses of IL-2 a second component of clearance with a half-life of 30-120 min was found. Acute toxicity was minimal and consisted of headache (6 of 12), nausea (4 of 12), malaise (6 of 12), and fever and chills (8 of 12). No evidence of pulmonary, hematologic, or renal toxicity or any evidence of autoimmune phenomena was detected. A transient hyperbilirubinemia was seen in two patients receiving 2 mg purified IL-2. No demonstrable effect on tumors or chronic immunodeficiency (AIDS) was seen. No consistent chronic immunologic effects (natural killer or lymphokine-activated killer activity, mitogen responsiveness, total lymphocyte counts, or change in the proportion of various mononuclear cell phenotypes as defined by monoclonal antibody) were seen on a week-to-week basis during or following therapy. Acute changes in lymphokine responsiveness, the ability to generate lymphokine-activated killers, and an increase in macrophages in the mononuclear population were noted following administration of 1-2 mg IL-2.