endotoxin:

a lipopolysaccharide component of the membrane of gram-negative bacteria that is heat stable and toxic; a secreted toxin produced by bacteria is termed an "exotoxin" 

The complex phospholipid-polysaccharide macromolecules which form an integral part of the cell wall of a variety of relatively avirulent as well as virulent strains of Gram-negative bacteria. The toxins are relatively heat-stable, are less potent than most exotoxins, are less specific, and do not form toxoids; on injection, they may cause a state of shock accompanied by severe diarrhea, and, in smaller doses, fever and leukopenia followed by leukocytosis.