THE FORMATION OF MILK SUGAR : THE IN VITRO SYNTHESIS OF LACTOSE BY ACTIVE MAMMARY GLAND PREPARATIONS.

1. The mammary glands of rats in the height of lactation were frozen in situ, removed, and dried in the vacuum in the frozen state. 2. The dried gland, added to pig serum, pig serum plus glucose, pig serum plus galactose, and pig serum plus lactose, then incubated at 37.5 degrees C., yielded a synthesis of non-fermentable, reducing material. 3. The dried gland added to glucose solution alone and incubated at 37.5 degrees C. yielded a synthesis of a non-fermentable reducing disaccharide, having a reaction velocity of reduction resembling that of pure lactose. 4. There is a "short time" storage of some forms of lactose precursor in the active mammary gland. This precursor is non-reducing but becomes available and reducing when the gland is placed in a solution containing glucose.