OUABAIN, C30H46O12, a crystalline glucoside occurring in the seeds of Strophanthus gratus and in the wood of various Acocanthera species,was first isolated from Acocanthera Schimperi by Arnaud in 1888 and named ouabain, from ouabaic, an arrow poison. It is described in the German Pharmacopœia (1926) and is included in the United States Pharmacopœia (1926) for use as a standard for the biological control of preparations of digitalis and strophanthus. It is particularly suited for use as a standard substance, since it is easily purified and the purity is readily ascertained by the ordinary methods of chemical analysis, so that the standard is easily reproducible. Ouabain for use as a biological standard is distributed by the Bureau of Standards in the United States, and has been recommended for use by the League of Nations Health Organisation. As a pure substance it offers decided advantages, and may eventually replace the by no means uniform strophanthin of commerce, which consists of a mixture of glucosides derived from Strophanthus Konibé, probably in many cases contaminated with other species not readily distinguishable. Ouabain has not hitherto been readily available commercially, but it has been prepared for a number of years past in the experimental laboratories of Messrs. Burroughs Wellcome and Co. for use in the associated research institutions. It has now been put on sale for research purposes and for use in clinical medicine.