Most adults of milk-drinking peoples can absorb lactose whereas those from ethnic groups without a tradition of milking cannot. This is related to the persistence or absence of lactase activity in the brush border of the enterocyte and appears to be determined genetically. As yet there is no conelusive evidence that major alterations of human lactase activity can be induced by dietary manipulation. In most studies of lactase-deficient peoples occasional adults are reported as able to absorb lactose. Studies of such individuals may help to determine factors controlling the production of brush border lactase.