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SUMMARY: The biochemical characteristics of approximately eighty strains of Friedläder’s bacillus are described. Certain aspects of their antigenic patterns were studied by testing the precipitating antigens of phase variants of originally mucoid strains, as well as the parent strains themselves. Evidence is presented that an as yet undescribed antigenic layer exists between those characteristic of the mucoid and smooth phases. This layer is most pronounced in the Sm phase organisms. The Sm antigen was shown to be present in nearly all Friedländer’s strains and absent in the Bacterium coli strains studied; hence an Sm antiserum should be valuable for diagnostic purposes. The biochemical and serological results are discussed in regard to the taxonomy of this group with particular reference to the consolidation, already proposed by other workers, of the genera Klebsiclla and Aerobacter.