The aim of this work was to improve radiotherapy results by immune stimulation. We tested the effects of a combination of radio-and immunotherapy, i.e., local low dose recombinant interleukin-2 (rIL-2) treatment, in two murine tumor models. Syngeneic tumors (SL2 lymphoma or M8013 mammary carcinoma) were induced subcutaneously on one or both flanks of mice. Irradiation was given either as a single dose (20 Gy) or fractionated (25 Gy) in 2 weeks. One or two cycles of rIL-2 were given concurrent with or subsequent to radiotherapy. One cycle of rIL-2 consisted of peritumoral injections administered on 5 consecutive days. Treatment effects were measured in terms of local tumor response and disease-free survival (DFS). The combined treatment modality was significantly better than treatment with either irradiation alone or rIL-2 alone. When tumors were inoculated on both flanks of the mice, combined radioimmunotherapy of one of the tumors also resulted in regression of the contralateral untreated tumor, indicating that a systemic anti-tumor immune reaction was induced. Additional rIL-2 injections did not enhance radiation toxicity. In conclusion supplementing irradiation with locally administered low doses of rIL-2 results in better local anti-tumor responses and DFS rates than either treatment alone without enhanced treatment toxicity. Furthermore, the local treatment induces a systemic anti-tumor reaction, influencing the growth patterns of a second, untreated tumor.