Experiments with fraction P of acetylated endotoxin, possessed of reduced toxicity, have demonstrated that it does provoke a mild Shwartzman phenomenon in some animals, probably owing to inferior solubility and a consequent blocking of the reticuloendothelial system. However, unlike the unchanged endotoxin, it only slightly raises the antibody titer and stimulates no skin hypersensitivity to the endotoxin. The hypersensitivity has the character of Arthus-type allergy associated with antibodies, and certain features of delayed allergy, which may conceivably depend on antigen-antibody complexes. In spite of the low antibody titer, the sensitivity induced with acetylated endotoxin can be transferred in some cases, as can that to the unchanged endotoxin.