Groups of male CBA/J mice were injected with Salmonella typhimurium lipopolysaccharide (LPS) and irradiated with 2450 MHz (CW) microwaves. The 50% lethal dose (LD50) of LPS was determined for mice irradiated at 30, 20, 10 and 5 mW/cm2 immediately following injection. The average specific absorption rate was approximately 0.6 W/kg per 1 mW/cm2 incident power. An equal number of animals served as shamirradiated controls for each power density. The mice were placed individually in small containers and were maintained at 22°C and 50% relative humidity during a 2 hour irradiation period. Following irradiation the mice were returned to their home cages and were observed for 48 hr. A significant decrease in the LPS dose required to kill 50% of the mice was observed at power densities of 20 and 30 mW/cm2. High ambient temperature (37°C) also potentiated the lethal effect of endotoxin. Microwave irradiation prior to LPS injection, however, did not affect the lethal action of LPS.