This type of fermented milk is produced by an acid-forming organism that can form large amounts of acid, 2.0 or 3.0 per cent. The casein is dissolved to some extent and the remainder so changed, that it will remain in suspension for a long time in a finely divided form, after the curd has been broken up. Such milk is sold under various names at home and abroad. One of the authors (H) has found such organisms in practically all milks examined. If raw milk is kept warm (98° to 100° F.) in a stoppered bottle which is filled full, the acidity will be found to increase slowly from day to day, reaching a maximum in ten to fourteen days. If the milk is then examined, it will be found to contain large numbers of an acid-forming organism very different in appearance from the bacteria causing the rapid souring of milk at ordinary temperatures. This organism is very similar if not identical with the one found in the Bulgarian milk to which the name _B. Bulgaricus_ has been given. The use of the milk fermented by this organism has spread rapidly because it is claimed by certain European bacteriologists that it has a favorable effect on the health of people, especially those suffering from intestinal troubles. It is not at all certain that ordinary sour milk or butter milk will not have the same effect; in fact in many of the fermented milks sold in Europe, _B. Bulgaricus_ has not been found, but only the ordinary lactic acid bacteria. Several alcoholic drinks made from milk, such as kefir and koumiss, have been originated among the nomadic tribes of Western Asia. Kefir is prepared from cow's milk by adding the kefir ferment in the form of grains which contain a number of kinds of bacteria and a yeast. The acid-forming bacteria impart a sour taste to the fermented milk, while the yeast forms carbon dioxide and about two per cent of alcohol. If the milk is allowed to ferment in stoppered bottles, the resulting product will be an acid effervescing drink, which is claimed to be more easily digested than sweet milk. This drink is used frequently in the treatment of invalids but it is improbable that it is more easily digested than ordinary soured milk or butter milk. The grains are removed from the fermented milk, and are then added to a quantity of fresh milk, or they may be dried and kept for future use. When needed again, they are soaked in water, then added to the milk. Koumiss is made in Russia from mare's milk and has much the same composition as kefir. In America and Europe it is made from cow's milk, by adding cane sugar and compressed yeast. The yeast ferments the cane sugar while the acid-forming bacteria ferment the milk sugar. There is thus obtained a drink that is similar in composition to the real koumiss, in which both the acid and the alcohol come from the fermentation of the milk sugar. In koumiss and kefir the curd is very finely divided and will remain in suspension for a long time as with butter milk.