A quantitative study of the serological cross-reactions of the somatic antigens of Types I, III, and I-III Shigella paradysenteriae has been made.

From the results obtained it has been concluded that the somatic antigens of Shigella paradysenteriae (Flexner) Types I, III, and I-III are single chemical substances. The specificity and serological cross-reactions exhibited by these antigens are dependent upon similarities in their chemical constitutions.

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