Purpose Interleukin-15 (IL-15) has significant potential in cancer immunotherapy as an activator of antitumor CD8 T and natural killer (NK) cells. The primary objectives of this trial were to determine safety, adverse event profile, dose-limiting toxicity, and maximum-tolerated dose of recombinant human IL-15 (rhIL-15) administered as a daily intravenous bolus infusion for 12 consecutive days in patients with metastatic malignancy. Patients and Methods We performed a first in-human trial of Escherichia coli–produced rhIL-15. Bolus infusions of 3.0, 1.0, and 0.3 μg/kg per day of IL-15 were administered for 12 consecutive days to patients with metastatic malignant melanoma or metastatic renal cell cancer. Results Flow cytometry of peripheral blood lymphocytes revealed dramatic efflux of NK and memory CD8 T cells from the circulating blood within minutes of IL-15 administration, followed by influx and hyperproliferation yielding 10-fold expansions of NK cells that ultimately returned to baseline. Up to 50-fold increases of serum levels of multiple inflammatory cytokines were observed. Dose-limiting toxicities observed in patients receiving 3.0 and 1.0 μg/kg per day were grade 3 hypotension, thrombocytopenia, and elevations of ALT and AST, resulting in 0.3 μg/kg per day being determined the maximum-tolerated dose. Indications of activity included clearance of lung lesions in two patients. Conclusion IL-15 could be safely administered to patients with metastatic malignancy. IL-15 administration markedly altered homeostasis of lymphocyte subsets in blood, with NK cells and γδ cells most dramatically affected, followed by CD8 memory T cells. To reduce toxicity and increase efficacy, alternative dosing strategies have been initiated, including continuous intravenous infusions and subcutaneous IL-15 administration.