Interleukin-1 has been reported to be an effective radioprotective agent in mice subjected to lethal doses of irradiation. Production of Interleukin-1 can be increased by whole body hyperthermia. Therefore, whole body hyperthermia was assessed for its efficacy in protecting the lethal effects of ionizing radiation in DBA/2 mice. One hour of 40 degrees C +/- 0.2 whole body hyperthermia given 20 hr before 900 cGy total body irradiation protected 100% of DBA/2 mice from an LD 100/16 radiation dose (dose of irradiation that killed 100% of the mice in 16 days). Lethal doses of total body irradiation produced profound monocytopenia, decreased cellularity of thymus, spleen, and bone marrow, and suppressed Interleukin-1 production. Interleukin-1 production was determined using the thymocyte proliferation assay. Whole body hyperthermia accelerated recovery of blood leukocytes by up to 5 days post-total body irradiation in DBA/2 mice. Thymocytes, spleen, and bone marrow cells were activated by whole body hyperthermia, as assessed by the cell's response to Concanavalin A. This was accompanied by accelerated Interleukin-1 generation. Our results provide the first evidence that whole body hyperthermia acts as a potent radioprotector in vivo, effects that may be mediated by Interleukin-1.