Russell's double-sugar medium was first described in 1911.1 This medium is a meat-extract agar, containing 0.1 fo of dextrose, l.Ofo of lactose, and an indicator. It is used in the form of an agar slant, with a long butt, the slant surface and the butt being about equal in length. The medium is inoculated by making a deep stab into the butt, then a streak along the slant surface. Russell's medium has found a wide application in the preliminary differentiation of members of the typhoid-colon group. When a colony of an organism belonging in this group is transferred to double-sugar medium, the following changes may be observed after 24 hours: (a) If the organism ferments dextrose and lactose with acid and gas, slant and butt are acid, and there are gas bubbles throughout the medium (for example, Bact. coli). (b) If the organism ferments dextrose with acid and gas and does not ferment lactose, the slant is alkaline, the butt is acid, and there are gas bubbles chiefly in the butt (for example, paratyphoids?intermediates). (c) If the organism ferments dextrose with acid, but no gas, and does not ferment lactose, the slant is alkaline, the butt is acid, and there are no gas bubbles (for example, typhoid?dysentery), (d) If the organism ferments neither dextrose nor lactose, the slant and butt are alkaline (for example, Bacillus alkaligenes). It is possible, therefore, by means of this medium, to determine the subgroup in which an organism belongs; its complete identification is thereby facilitated. More recently, Krumwiede and Kohn2 have advocated the addition of 1.0% of saccharose to Russell's medium for the purpose of detecting certain members of the intermediate group which ferment dextrose and saccharose with acid and gas, but which ferment lactose slowly or not at all. On Russell's medium these organisms might be mistaken for members of the paratyphoid group.