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SUMMARY: Strains of Aerobacter aerogenes, A. cloacae and Escherichia coli, includingmucoid capsulate, mucoid non-capsulate, smooth and rough variant forms, were grown on defined agar media. The polysaccharide content of the cultures was estimated by the anthrone method and related to the amount of growth as measured by the non-dialysable nitrogen content. Polysaccharide production, as measured by the polysaccharide : nitrogen ratio, was increased in all strains when growth was limited by a restricted supply of nitrogen source so that added carbohydrate remained in relative excess. A low incubation temperature, 15-20° instead of 3 5 O , influenced polysaccharide production differently in the various strains and species ; thus it greatly increased production in E. coli strains, but decreased it in A. cloacae strains grown on certain media. The various strains showed considerable differences in the amount of polysaccharide production and in the distribution of the polysaccharide between the different possible sites of accumulation : i.e. intracellular, capsular and as loose slime. The mucoid strains produced the largest amounts of polysaccharide, most of which was present extracellularly as capsuIes or slime; the smooth strains produced much smaller amounts, almost entirely intracellular; a rough strain produced least of all. The antigenic constitution of certain strains was examined, particularly that of a mucoid capsulate A. aerogenes and mucoid non-capsulate and non-mucoid smooth variants derived from it. The designation of such morphological and immunological variants is discussed.The amount of polysaccharide produced varies greatly with different strains and species of Enterobacteriaceae. Among twelve strains of Escherichia coli grown at 13-17' on glucose medium, Morgan & Beckwith (1939) found that the amount of polysaccharide produced, as indicated by the degree of mucoidness, was large in five strains, moderate in two strains and small in the remainder ; they found similar differences among salmonella strains. Mucoid, smooth and rough variants of the same strain may also show great differences in polysaccharide production. (1951), using the periodate-Schiff staining method, found that 0-inagglutinable rough variants of salmonella and shigella strains lacked the stainable intracellular polysaccharide of the smooth parent strains ; they therefore suggested that this polysaccharide corresponded to the hapten of the 0 antigen characteristic of the smooth form.Duguid & Wilkinson (1953) studied the influence of different cultural conditions on the amount of polysaccharide produced by a single strain (A3) of Aerobacter aerogenes, distinguishing the polysaccharide situated intracellularly, in the capsules or in the loose extracellular slime. In the present investigation the same methods have been applied for examination of several different strains of A . aerogenes, A . cloacae and Eschrichia coli. For each strain the effect was observed of variation in the sugar:nitrogen ratio of the medium and of variation in th...