We analyzed the mechanism of the augmentation of endotoxin fever by human beta interferon (IFN) cross-reacting to rabbit cells in rabbits by using a purified rabbit tumor necrosis factor (RaTNF) and a monoclonal anti-RaTNF. The late peak of fever evoked by the injection with both endotoxin and HuIFN was suppressed when the animals were injected previously with anti-RaTNF. IFN also augmented the pyrogenicity of RaTNF in a synergistic manner in rabbits. The blood collected 2 h after the injection of RaTNF plus IFN contained a significant endogenous TNF activity, and the serum was shown to be pyrogenic. The endogenous pyrogen activity in the 2-h blood was heat stable (70 degrees C, 30 min) and was reduced by the in vitro treatment with anti-RaTNF. These results suggest that IFN augments the febrile response of rabbits to endotoxin by stimulating endogenous TNF-mediated TNF production to induce the late peak of fever.