Interleukin 1 (IL-1) is a monokine secreted by macrophages which causes some of the symptoms associated with acute inflammation such as fever and the production of acute phase reactants. We previously showed that aqueous extracts of cotton can stimulate IL-1 production, but that the activity appeared to be independent of the endotoxin content of the extracts. To further clarify this point we have examined the ability, of endotoxins derived from different bacterial species to induce IL-1 production. The results indicate that the IL-1 inducing activity is associated with the lipopolysaccharide (LPS) portion of the endotoxin molecule, and that LPS isolated from Enterobacter agglomerens (kindly provided by Cotton Incorporated, Raleigh, NC) is more potent stimulator of IL-1 production than E. coli LPS. These results may explain why the endotoxin content and IL-1 inducing activity of cotton extracts do not correlate.