The aim of this phase I study was to exploit the potential efficacy of an alpha-2a-interferon (alpha-2a-IFN)-subcutaneous interleukin-2 (IL-2) combination, bypassing the toxicity usually associated with bolus or continuous infusion of IL-2. Therefore, nineteen patients with metastatic malignancies (7 melanomas, 6 renal cell carcinomas and 6 soft tissue sarcomas) were treated according to a dose escalating schedule of subcutaneous IL-2 combined with intramuscular alpha-2a-IFN for 5 days/week for 3 consecutive weeks. Cycles were repeated every 2-4 weeks unless disease progressed. Alpha-2a-IFN (3 MU/die) was given continuously, including during the rest weeks. IL-2 doses were started at 2 MIU/day/sqm and the MTD of 6 MIU/day/sqm was progressively reached. The dose of IL-2 was given twice daily every 12 hours. Both of the cytokines were administered in an outpatient setting. The main side effects were fever, chills, fatigue, hypotension, nausea and vomiting. Toxicity was correlated with IL-2 dose level. It was found to be mild at 2 and 4 MIU/day/sqm, while, in contrast, grade III toxicity was observed only at the highest dose of 6 MIU/day/sqm. However, this grade III toxicity was manageable and did not prevent continuation of the treatment as long as the dose was not increased above 6 MIU/day/sqm. Three patients, one with melanoma and two with renal cell carcinomas, obtained clinical partial responses. In eight patients, stable disease, and in the remaining eight, progression, were observed. The data suggest that the combined use of the two BRMs has manageable side effects and would seem to be efficacious. A phase II study at the recommended dose of 6 MIU/day is now necessary.