Contact of a purified endotoxin from Shigella fiexneri Type Z with normal rabbit or human serum results in an alteration of its immunological reaction with antiserum, as determined by precipitation in gel. Analysis of fractions of normal serum obtained by zone electrophoresis in starch indicates that the component responsible for altering the immunological reaction of endotoxin is associated with beta globulin. Normal serum has no similar effect on the immunological reaction of a variety of other protein and polysaccharide antigens. Serum from rabbits made tolerant to the pyrogenic action of an endotoxin from Serratia marcescens (P-35) possesses the ability to alter the reaction of Shigella endotoxin with its specific antiserum, although the serum from tolerant rabbits does not significantly enhance the pyrogenicity of Shigella toxm. The component of normal rabbit serum responsible for the effect on the immunological reaction of Shigella endotoxin is not destroyed by heating at 56°C. The possible relationship of the effect of normal serum on the immunological reaction of endotoxin and the augmentation of fever induced by endotoxins is discussed.

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