The endotoxins of Gram-negative bacteria, sometimes referred to as the "bacterial pyrogens," are the macromolecular lipopolysaccharide complexes which comprise the somatic antigens of these bacteria. Although distinctly different from each other with respect to antigenicity, depending on differences between the polysaccharides of various species, they are remarkably similar in the physiologic and pathologic reactions which they produce in animals. According to Westphal,1 the lipid component is responsible for toxicity and is common to all endotoxins. The complexity of the response to endotoxin is illustrated by a partial list of the mechanisms brought into play by a single injection: fever, leukopenia followed by leukocytosis, peripheral vasoconstriction and vasomotion, shock, depletion of liver glycogen, hyperglycemia, increased adhesiveness of polymorphonuclear leukocytes, heparin-precipitability of fibrinogen, impeded phagocytosis by reticuloendothelial cells, augmentation of antibody response to protein antigens, abortion, hemorrhagic necrosis in malignant tumors, and disturbances of the reactivity of terminal blood vessels to