We reported recently that the onset of lipopolysaccharide (LPS)-induced fever, irrespective of its route of administration, is temporally correlated with the appearance of LPS in the liver, and that splenectomy significantly increases both the febrile response to LPS and the uptake of LPS by Kupffer cells (KC). To further evaluate the role of the spleen in LPS fever production, we ligated the splenic vein and, 7 and 30 days later, monitored the core temperature changes over 6 h following the intraperitoneal (ip) injection of LPS (2 µg/kg). Both the febrile response and uptake of LPS by KC were significantly augmented. Like splenectomy, splenic vein ligation (SVL) increased the febrile response and LPS uptake by KC until collateral circulation developed, suggesting that the spleen may normally contribute an inhibitory factor that limits the KC uptake of LPS and thus affects the febrile response. Subsequently, to verify the presence of this factor, we prepared splenic extracts from guinea pigs pretreated with LPS (8 µg/kg, ip) or pyrogen-free saline, homogenized and ultrafiltered it, and injected it intravenously (iv) into splenectomized (Splex) guinea pigs pre-treated with LPS (8 µg/kg, ip). The results confirmed our presumption that the splenic extract from LPS-treated guinea pigs inhibits the exaggerated febrile response and the LPS uptake by the liver of Splex guinea pigs, indicating the presence of a putative splenic inhibitory factor, confirming the participation of the spleen in LPS-induced fever, and suggesting the existence of a novel antihyperpyretic mechanism. Preliminary data indicate that this factor is a lipid.