Recombinant human (rhu) interleukin 2 (IL-2) was evaluated alone and in combination with local hyperthermia (LH) in mice inoculated s.c. with 5 x 10(5) Lewis lung carcinoma cells. Four treatment regimens were begun 6 days postinoculation at a time when the tumor had grown to approximately 8.0 mm in diameter. Treatments were: group 1, saline injected as control; group 2, LH; group 3, rhuIL-2; or group 4, LH combined with rhuIL-2. LH utilized hot water circulation by a Brann Thermomix 1420. The intratumor temperature was maintained at 43 +/- 0.2 degrees C for 30 min each on days 6 and 10 and rhuIL-2 was given s.c. at 5 x 10(4) units twice a day for 5 days. Thirty mice in each group were sacrificed 28 days after tumor inoculation. An additional 20 mice in each group were observed for survival time. The size of primary tumor and the number of lung metastases were reduced and the survival time was prolonged in mice treated by either LH or IL-2. However, a greater antitumor effect in Lewis lung carcinoma tumor-bearing mice was observed using IL-2 therapy combined with LH. Tumor growth was associated with increased splenic granulocyte-macrophage progenitor cells and an abnormal L3T4+/Lyt-2+ lymphocyte subset ratio (less than 1.0). Splenic granulocyte-macrophage progenitor cell numbers and the L3T4+/Lyt-2+ ratio returned to normal in the group treated with combination therapy, the best responder group. The L3T4+/Lyt-2+ ratio did not change in the groups treated with single therapy. These results suggest the efficacy and possible clinical relevance of combined therapy with rhuIL-2 and LH for certain metastatic tumors.