Summary: In a serological study of the mannitol fermenting, lactose negative paradysentery group of bacteria, type-specific antigens for types V, W, Z, 88, 103 and P119 in the Flexner group were identified by means of specific agglutinins of sufficient strength to provide a practical means of strain identification. Specific antigens also were readily identified in cultures of Boyd types 170, P288, P274, D1, D19, P143. In the case of X and Y types only weak specific agglutinins were obtained and it seems likely that these cultures are degraded types as Boyd has proposed. Strains of types 170 and P274 isolated in the U. S. appeared to be identical with the stock strains. Extension of serological identification to include the less common Boyd types, 170, P288, P274, D1, D19 and P143, will provide a means of studying the significance of these strains in dysenteric disease. Major group-antigens were identified among types in the Flexner group which were of sufficient importance for use in strain identification. By adsorption, serums were obtained showing group-components common to V-Z-103, V-W-Y-88, W-Y, V-88, X-Z-P119-W1268, X-Z-W1268, V-P119 and V-W1268-X-Z-P119 cultures. The same type-specific antigen was found in combination with more than one set of group-antigens, e.g. the stock strain of type 103 had the major groupantigen of V and Z cultures while other strains of 103 lacked this but shared an antigen with the W and Y types. Different group-antigens were combined with the same specific antigen in the case of W strains, one sharing a major group-antigen with V, Y and type 88 strains the other with X, Z, and P119 strains. One type P119 culture was found which had some of the antigen of W-Y strains in addition to the principal group-antigen of the stock P119 strains. The classification of Flexner types as outlined by Boyd provides a practical method of classification which has the advantage of pointing out the close serological relationships among the Flexner types. Some modifications based on the distribution of group-components among the types are desirable. Many of the broad antigenic relationships are of no differential value. Although Boyd types 170, P288, P274, D1, D19 and P143 possess little or none of the “Flexner” group-antigen the scheme of classification can eventually be extended to include these types.