On the pyrogenic action of intravenous lipid A in rabbits.

1. Previous evidence purporting to show that lipid A is the pyrogenic moiety of endotoxin is demonstrably inconclusive. 2. We have extracted lipid A from endotoxin of Salmonella typhosa and tested the pyrogenic action of the lipid A, the residual polysaccharide and the parent endotoxin, by intravenous injection in conscious rabbits. 3. Lipid A dissolved in an aqueous solution of rabbit serum albumin induced a significant pyrexia of short latency, while neither rabbit serum albumin alone, nor the polysaccharide from S. typhosa, affected body temperature. The physical presence in the injectate of the polysaccharide from S. typhosa did not enhance the pyrogenicity of the lipid. 4. Dose-response curves for lipid A and the parent endotoxin, over the dose range 10 ng-20 micrograms, showed that lipid A incorporated in endotoxin was much more pyrogenic than pure lipid A in solution. When separated from the polysaccharide component of endotoxin, lipid A lost more than 99.9% of its pyrogenic activity, at threshold doses.