The authors compared by X-ray the passage of a milk containing lactose with that of a lactose-free milk in subjects after partial gastrectomy who, after operation, developed milk intolerance. In the X-ray picture of subjects with a lower mucosal lactase level, typical differences between the two passages are found. The main difference is that the gastric stump is unable to slow down the milk supply sufficiently to the small intestine and, thus, as a result of the lower mucosal lactase level, the relative lactase insufficiency becomes manifest. During the fractionated administration of the lactose containing milk mixture the X-ray intolerance changes disappear. When the X-ray pictures after both milk mixtures do not differ, milk intolerance has obviously another cause than lactose maldigestion.