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Amoebae of the cellular slime mold, Dictyostelium discoideum, were grown in submerged culture on the following bacterial strains which differ with respect to the sugar composition of their somatic antigens (lipopolysaccharides) : Salmonella london S form containing Lipid A, 2-keto-3-deoxyoctonate (KDO), heptose, glucose, galactose, mannose and rhamnose ; the R mutant Escherichia coli B/r, containing Lipid A, KDO, heptose, and glucose; and the R mutant Salmonella minnesota R595 containing Lipid A and KDO. While the two R strains were phagocytized spontaneously by the amoebae, the S strain S. london had to be opsonized by specific antiserum to promote phagocytosis. The amoebae were harvested either at the end of the exponential growth phase or in the aggregation competent stage of their development, i.e. 8 to 9 hours after exhaustion of the food bacteria. The yields of amoebae were independent of the bacterial strain used for cultivation and ranged from 30 to 40 g per 100 g of bacteria (dry weight).Amoebae and supernatant media (after dialysis) were extracted by the phenol-water method and the water-soluble extracts obtained were fractionated by centrifugation a t 100,000 x g (sediment : S fraction; supernatant : L fraction). No significant yields of lipopolysaccharide were obtained either from amoebae or from the culture media. However, substances with the serological specificity of the lipopolysaccharides of the food bacteria were detected in the L fractions of the amoebae during the growth phase and of the culture medium after food exhaustion. Gel filtration of these fractions as well as gaschromatography and quantitative determinations of their sugar constituents showed that these specific substances represent the polysaccharide moiety of the bacterial lipopolysaccharides. ,fLHydroxymyristic acid which is a main constituent of lipid A was absent from the polysaccharides and could also not be detected in amoebae or culture media.Besides bacteria-specific polysaccharides the amoebae contained additional polysaccharides with sugars which are not present in the food bacteria, such as L-fucose and D-mannose. These amoebae-specific substances are the subject of further studies.