For the first time, an 0-antigenic lipopolysaccharide (LPS) has been isolated from a filamentous blue-green alga (Anabaena variabilis). It was extractable with phenol-water, resulting in extraction of the bulk of the LPS into the phenol phase. The polysaccharide moiety of this LPS consists of L-rhamnose, its 3-0-methyl ether L-acofriose, D-mannose, D-glucose, and D-galactose. L-Glycero-D-mannoheptose and 2-keto-3-deoxyoctonate, the two characteristic sugar components of enteric LPS, and phosphate groups are absent from the A. variabilis O antigen. The only amino sugar present is D-glucosamine. Three hydroxy fatty acids were identified, namely, ,-hydroxymyristic, ,-hydroxypalmitic and B-hydroxystearic acids, in addition to palmitic and unidentified fatty acid. The LPS of A. variabilis is localized in the outermost cell wall layer and behaves like a bacterial 0 antigen in serological tests. The passive hemagglutination yielded high titers with isolated LPS (pretreated by heat or by alkali) and rabbit antisera prepared against living or heat-killed cells. The position of the precipitation arcs after immunoelectrophoresis of the 0 antigen indicates the lack of charged groups. The water phase of the phenol-water extract contains, in high yield, a glucose polymer. It is serologically inactive as shown by the passive hemagglutination test and by agar-gel precipitation.

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