We investigated the antitumor effects of combined immunotherapy with recombinant human interleukin-2 (rhIL-2) and the recombinant human interferon-α (rhIFN-α) A/D hybrid in the treatment of established single or multiple murine hepatic metastases. Mice bearing either weakly immunogenic MCA-106 or nonimmunogenic MCA-102 were treated with rhIL-2 alone, rhIFN-α alone, or the combination of lymphokines. Therapy was initiated on Day 3 or 10 and continued for 3–4 consecutive days. In the treatment of 3- and 10-day multiple MCA-106 liver metastases, significant reductions in the number of metastases, often more than 90%, were observed with the combination of rhIL-2 and rhIFN-α at doses of each lymphokine which had no effect when given alone. This decrease in the number of metastases resulted in a survival benefit that was seen in the combination therapy groups in a dose-dependent manner. Similarly, substantial reductions in tumor weight were seen when the combination of rhIL-2 and rhIFN-α was administered to mice with single large hepatic metastases. The decreases in both single and multiple metastatic tumor deposits by the combination of lymphokines were more than that predicted by the additive effect of each treatment alone. With the nonimmunogenic tumor, MCA-102, however, no benefit was derived from the addition of rhIFN-α to rhIL-2 therapy. Immunotherapy with recombinant murine interferon-γ and rhIL-2 was directly compared to therapy with rhIL-2 and rhIFN-α. The combination of rhIL-2 and rhIFN-α again was found to be effective while recombinant murine interferon-γ added toxicity but no therapeutic benefit to immunotherapy with rhIL-2 alone. The synergy between rhIL-2 and rhIFN-α was shown to be dependent on the host's intact immune system since mice immunosuppressed by sublethal irradiation prior to inoculation of tumor did not respond to the combined treatment. Possible mechanisms of the in vivo synergy between rhIL-2 and rhIFN-α are discussed.