Fifteen patients with acquired immunodeficiency syndrome (AIDS), lymphoma and immunodeficiency, or severe combined immunodeficiency were treated with highly purified interleukin-2 (IL-2) prepared from human lymphocytes. All patients showed a defect in mitogen-induced T cell proliferation which was partially corrected when IL-2 was added in vitro. IL-2 was administered subcutaneously by daily injection or continuous infusion. The maximum daily dose was 20,000 U/m2, the maximum total dose 855,000 U/m2, and the maximum period of treatment 77 days. An increase in the platelet count was seen in one patient with AIDS, a decrease in the serum level of a monoclonal immunoglobulin in another patient with AIDS, and a minor tumor response in a patient with diffuse histiocytic lymphoma. As no toxicity was observed, further study of IL-2 in the treatment of human immunodeficiency is indicated.