1. A bacterial toxin not freely liberated into the surrounding medium, in contrast to exotoxin. 2. The complex phospholipid-polysaccharide macromolecules that form an integral part of the outer membrane 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 and, in smaller doses, fever, and leukopenia followed by leukocytosis; they have the capacity of eliciting the Shwartzman and the Sanarelli-Shwartzman phenomena.