Microorganisms belonging to the group Shigella paradysenteriae (Flexner) are of considerable importance from an epidemiological point of view, and hence we have undertaken a study of their specific antigens. In this and subsequent reports the isolation and properties of the antigens of several types of Flexner dysentery bacilli are given together with an account of their immunological characteristics.The isolation of cell-free antigenic constituents from Gram-negative microorganisms has been reported by a number of investigators; most notable among these are Boivin (1) and his collaborators who have studied the specific antigens of Salmonella typhiraurium, Salmonella enteritidis, Eberthella typhosa, Escherichia coli, etc., and Topley (2) who has described the specific H and O antigens obtained from enzymatic breakdown products of E. typhosa. Perhaps the most comprehensive study regarding the nature of antigenic fractions from Gram-negative bacilli is that of Morgan and Partridge who have studied in detail the specific antigens of the Shiga (3) and typhoid (4) bacillus and have investigated their chemical and immunological properties.The chemical nature of antigens derived from Gram-negative bacilli is not yet fully understood. Boivin regarded these substances as belonging to a class of compounds known as lipocarbohydrates. The early investigators failed, however, to characterize a protein or protein-like constituent which was shown by later workers to form a part of the type specific antigens derived f~om the Shiga and typhoid bacillus. I t remained for Morgan and Partridge (3) to demonstrate that this protein forms an important constituent of the antigenic complex of both organisms and it has many unusual chemical and immunological properties. EXPERIMENTALCultivation of Bacteria.--The types V, W, Z, Shigella paradysenteriae (Flexner) used in this study were obtained from the U. S. Army Medical Center. The Newcastle strain was sent us by Dr. Carl TenBroeck of Princeton. The organisms were grown in a broth consisting of 50 gm. of bactotryptone, 100 cc. of blood extract (5), 400 gm. Na~HPO4.12 H20, 100 gm. KH2PO,, and 25 liters of tap water. Copious growth was obtained in 18 hours. As a rule, 50 liters of culture medium yielded 25 gin. of dry organisms. The living bacteria were col-

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