Our experiments demonstrate the relative importance of the various fractions derived from the complete antigen of S. dysenteriae in the production of various endotoxic reactions. The lethal effect, the decrease in liver glycogen, the decrease in ascorbic acid in the adrenals and the haemorrhagic reactions are closely associated with the presence of the phosphorus complex attached to the protein. The fractions of the complete antigen which do not contain this phosphorus-group, (such as the polysaccharides, the phospholipid and the simple protein) are not able to produce any of these effects. But the complete antigen (“A”), in which the conjugated protein is linked with the carbohydrate and the phospholipid complex, is more toxic than the conjugated protein (a2) after its isolation from the other constituents of the O-antigen. Another effect of the endotoxin, leucopenia, is associated with the presence of either of the phosphorus-groups (that attached to the protein or that attached to the carbohydrate). This reaction was produced by the conjugated protein as well as by the undegraded polysaccharide, while the corresponding simple compounds to which the specific phosphorus groups were not attached were inactive. The Shwartzman phenomenon was produced only in the presence of the phosphorus group attached to the carbohydrate either as a “preparing” or an “injury-producing” factor. The fact that the phosphorus-group attached to the protein is not active in this respect was unexpected. In his comment on the experiments of Olitzki and Leibowitz (21), Shwartzman (15) emphasizes the point that it is somewhat dangerous to classify the potency of bacterial fractions on the basis of the size of the lesion, which depends not only on the potency of the material but also on its ability to diffuse into the injected site. The results presented in table 7 show that this consideration may be of importance. The easily soluble complete antigen (“A”) produced more extensive lesions in all instances than did the relatively less soluble protein-carbohydrate complex (a4); and the most extensive lesions were produced by the undegraded polysaccharide (a6) which yielded clear, slightly viscous aqueous solutions. In contrast to the other endotoxic reactions the hypothermic reaction is not associated with any distinct fraction. Especially in the highly sensitive rats and mice, hypothermic reactions were produced by injections of either proteins or carbohydrates (conjugated and simple), the only inactive fraction being the phospholipid. The degraded polysaccharide lost its activity after treatment with NaOH. In the less sensitive guinea-pig, however, the hypothermic reaction was produced only by the conjugated protein and the undegraded carbohydrate, a fact which again demonstrates the importance of the phosphorus-complexes. The pathogenizing activity of the complete antigen may be ascribed mainly to the polysaccharides, and to a lesser extent to the conjugated protein. The importance of the phosphorus group is here also obvious, since the undegraded polysaccharide was more active than the degraded one, and the conjugated protein was active while the simple protein was not. The antigenic properties of the different fractions were studied by Morgan et al. (9–13); the experiments reported here confirm the results obtained by these authors.