In Drosophila, the Toll family of proteins mediates the innate immune response. Toll is activated by Spaetzle, which is generated in response to pathogens via a serine protease cascade. We wished to investigate if lipopolysaccharides (LPS) might activate Toll-like receptor (TLR) 4 via a serine protease in humans. The serpin antithrombin III (ATIII) and the thrombin inhibitor hirudin both inhibited nuclear factor (NF)-κB activation by LPS and Lipid A. ATIII and hirudin were also able to inhibit LPS-induced NF-κB activation in cells stably transfected with TLR4. These results suggest that LPS may activate a mammalian serine protease, which generates a product required for TLR4 signalling.