SUMMARYStudies performed on 10-day-old immunologically runted mice show normal intestinal transport of monosaccharides and an amino acid, but a marked deficit of intestinal lactase activity. Surprisingly, intestinal sucrase activity is prematurely increased in the runt. It is suggested that lactase deficiency leads to malabsorption of lactose (the main carbohydrate in mouse milk) and acid diarrhea, thus contributing to the pathogenesis of wasting in the infant mouse.