The effects of the aqueous phase of human breast milk on the disaccharidase activity of newborn rabbit small intestinal mucosal explants were studied in vitro culture. These explants continuously synthesized protein and normal morphology was maintained for the duration of the cultures. Addition of the aqueous phase resulted in significant increase of lactase (p / 0.001) and maltase (p /0.01) concentrations in these organ cultures. This effect was dose dependent and was observed whether the organ biopsies were derived from fed or starved newborn rabbits. Further purification of the aqueous phase showed that the active ingredient exerting these effects was lactose. These studies suggest that lactose may have an important function in stabilization of newborn intestinal disaccharidase enzymes.