Based on a preclinical study demonstrating the synergistic antitumor effect of recombinant interleukin 2 (rIL-2) and beta-interferon (IFN-beta) on mouse tumors and previous results of a phase I study of rIL-2, a phase I study of combination therapy with human rIL-2 and IFN-beta was conducted in 26 patients with advanced malignancy. Patients were given rIL-2 by 24-h continuous i.v. infusion and IFN-beta by 2-h i.v. infusion for 5 days each week for 4 weeks. The common side-effects were fever, malaise, chills, appetite loss, and diarrhea. Leukocytosis and eosinophilia were observed in 56% and 69% of the patients, respectively. Transient leukopenia and thrombocytopenia were also observed in some patients. Dose-limiting manifestations were intolerable fatigue and liver dysfunction, and it was concluded that the maximum tolerated doses of rIL-2 combined with IFN-beta were 1.1 x 10(6) U/m2/day for rIL-2 and 6.0 x 10(6) IU/m2/day for IFN-beta. No patients achieved complete and partial response to therapy in this study. One patient with pulmonary metastasis from pharyngeal cancer showed a minor response. Natural killer (NK) and lymphokine-activated killer (LAK) activities increased during the 5 days of treatment and decreased during the 2-day intermission. The percentage of IL-2 receptor-positive cells increased markedly until Day 12, and gradually decreased thereafter. The percentage of OKT 4-positive cells and the OKT 4/OKT 8 ratio increased. In contrast, the percentage of Leu 7- or Leu 11-positive cells decreased over the 4-week treatment. A phase II study of this combination therapy is ongoing against head and neck cancer, and renal cell carcinoma.