An antigen common to Enterobacteriaceae and closely associated with endotoxin fractions has been separated by chromatography on DEAE cellulose employing elution with a NaCl gradient. The purified common antigen fails to coat erythrocytes, is poorly, if at all antigenic, it is non-dialyzable and excluded from sephadex G-100 gel. It is composed of polysaccharide and polypeptide. The most important property of this antigen thus far determined appears to be its interference with the specificity of the hemagglutination test commonly employed to measure antibody to O antigen of Enterobacteriaceae. It may also have taxonomic significance in classification of this family of bacteria.

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