BalbC mice were innoculated with live L1 sarcoma cells and either sham exposed or exposed on day 14 after tumor implantation with 2450 MHz (CW) RF for 2 hours/day for 7 days at 50 mW/cm2 (average) in the far field of a horn antenna. In addition, some of the mice were immunized twice (at 14 day intervals) with heat killed L-1 sarcoma cells 17 days before live tumor implantation and exposed as above beginning 10 days after the final immunization, and then injected with live tumor cells. Exposure resulted in a rise of rectal temperature to 41.5 +/- 0.5 C. Sham mice received non-RF heating for 7 days. In non-immunized mice, microwave hyperthermia for 7 consecutive days resulted in temporary tumor regression, prolonged mean survival time (53 days vs. 38 days in controls), and more lung metastases. In non-immunized animals exposed to microwave hyperthermia before tumor implantation, tumor growth was accelerated, survival time was shorter, and there was an increased number of lung metastases. Microwave hyperthermia after immunization abrogated the beneficial effect of the microwave treatment on tumor growth as well as the beneficial effects of preimmunization. Although hyperthermia treatment appeared to prolong survival time and cause tumor regression in tumor bearing mice, and this was interpreted as due to the heat sensitivity of the tumor cells. When L-1 tumor cells were injected into hyperthermic RF treated mice, there was an apparent increase in metastatic progression that the authors interpret as a result of depressed anti-tumor immunity.