Lactose is a prime constituent of "the perfect food," milk. The normal nursling thrives on it and the healthy baby's intestinal flora is the envy of an everincreasing proportion of the adult population. Lactose is as much a food for beneficial microbes as for man.1 In the intestinal flora of the breast-fed baby, Bacillus bifidus predominates. When the baby is weaned on cow's milk, Bacillus acidophilus becomes the predominating microbial species. Lactose, as is well known, is an excellent source of energy for this microbe. It was Metchnikoff's belief that many human ills arise from harmful bacteria present in the large intestine of man and, further, that senility is causally related to this "useless" portion of the gastro-intestinal tract. Consequently, he concluded that a favorable intestinal flora was the summum bonum. This doctrine proved a stimulus to the devising of ways and means for the development in adults of