Mice infected with the polycythemia-inducing strain of Friend virus complex (FVC-P) develop a fatal erythroid disease similar in some respects to leukemia. Six- to eight-week-old DBA/2 female mice were injected i.v. with 0.5 ml of a virus suspension containing approximately 5 × 104 plaque-forming units and 5 × 103 spleen focus-forming units. Four treatment regimens were begun 3 days postinjection: (a) no treatment; (b) whole-body hyperthermia (WBH) alone; (c) cyclophosphamide (CY) alone; (d) WBH combined with CY. WBH treatment utilized a microwave generator operating at 2450 MHz. The i.p. temperature of the mice receiving WBH was maintained at 39.5–40°C for 30 min. The CY was given i.p. at a dosage of 20 mg/kg of body weight. The various treatments, CY, WBH, CY + WBH were given once a week for 2 weeks. Natural killer cell activity was examined in all four groups of mice and was found to be significantly higher in the animals treated with WBH or CY. Our results show that WBH, either alone or in combination with CY, can prolong the lifespan of mice infected with lethal dosages of the FVC-P, possibly via a mechanism involving natural killer cells.