A multi-lumen intubation system was used to study the absorption of calcium, glucose and galactose in 13 human subjects. The intubation was placed between the duodenum abdomen and proximal jejunum and the subjects were perfused with milk and lactase-supplemented milk. Lactose disappearance over a 20 cm length of intestine was used as the index of lactase activity. The subjects were assigned to one of two groups, lactase-normal and lactase-deficient. There was linear correlation between the absorption of calcium and lactose: lactase-deficient subjects absorbed less calcium than lactase-normal subjects. Perfusion with lactase-supplemented milk enhanced calcium absorption in lactase-deficient subjects but had no effect on that of normal lactase subjects. All subjects absorbed approximately the same percentage of perfused calcium (24%) when perfused with hydrolysed milk. These data indicate that the enhancement of calcium absorption is not a function of lactase per se, but of its hydrolytic products, glucose and galactose.