The best-characterised exogenous pyrogens are found in bacterial cell walls. 'Endotoxin' is a hydrophobic lipopolysaccharide which is a component of the cell walls of Gram-negative bacteria. Biological activity lies in the lipid A moiety, and the most important fatty acid producing fever has been identified as β-hydroxymyristic acid. As well as provoking the febrile response, endotoxin also plays a major role in the induction of septic shock. The molecule is widely distributed in Gram-negative bacteria and is found in non-virulent and weakly virulent species as well as in major pathogens such as salmonellae.