Experience with the administration of high doses of interleukin 2 (IL-2) alone is described herein. Ten patients with a variety of malignant disorders unresponsive to conventional treatments were treated with at least 30,000 U/kg of IL-2 by bolus administration three times a day. Patients were treated intravenously or intraperitoneally from four to 21 days in a single course, usually interrupted by a week of recovery. Three of six patients with melanoma experienced an objective regression (greater than 50% decrease in volume); there was no response to treatment in patients with colorectal (0/3) or ovarian (0/1) cancer. Two patients with initial objective regressions who subsequently developed progression were re-treated and one sustained a second partial response. Responses lasted 1, 3, and 7 months without additional treatment. Responses in the three patients with melanoma were in visceral sites (lung, liver, and spleen), as well as cutaneous sites in one patient. Progressive shrinkage of tumors for three to six months after the conclusion of therapy has been noted in two patients. Marked lymphocytic infiltrate was noted in a patient with lesions accessible to repeated biopsies. This study demonstrates that the administration of IL-2 can mediate the regression of established cancer in some patients.