Sixteen patients with metastatic melanoma or metastatic renal cell carcinoma were treated with six weekly 24-hour infusions of recombinant interleukin-2. At least three patients were treated at each dose, beginning at 3.0 mU/m2 for 24 hours each week for 6 weeks. Subsequent patients were treated at 4.5, 6.0, 8.0, and 10.0 mU/m2 for 24 hours. The incidence of diarrhea, rigors, rash, edema, and symptomatic hypotension was positively correlated with dose level. Symptomatic hypotension was dose limiting at the 10-mU/m2 level. Fever, nausea, and vomiting were seen at each dose level and could not be correlated with dose level. Lymphopenia and eosinophilia were observed at the completion of each 24-hour infusion, and an increase in peripheral blood absolute lymphocytes and eosinophils was observed over the 6-week treatment period. No thrombocytopenia was observed. No change in delayed-type hypersensitivity (type IV) as determined by skin testing could be demonstrated at any dose level. Natural killer cell cytotoxicity of peripheral blood lymphocytes increased over the treatment period, but the increase was unrelated to dose level in the range studied. One minor response was documented in a patient with renal cell carcinoma.