The cellular transport and oxidation of lactose-14C was studied in peroral jejunal biopsy specimens of normal and isolated lactase-deficient adults. The mucosal cells of subjects with isolated lactase deficiency could not concentrate lactose against a concentration gradient and demonstrated reduced oxidation of lactose to CO2. The small amount of radioactivity entering the cells was virtually all lactose in the lactase-deficient mucosa, whereas lactose accounted for only a small amount of the intracellular radioactivity in the normal mucosa. Galactono-γ-lactone and sodium-free medium, which inhibit lactase activity, suppressed the intracellular accumulation of lactose and its metabolic products in normal mucosa. These studies suggest that the jejunal cell transport of disaccharides does not occur in the absence of hydrolysis and that no specific permeases exist which transport the dissacharides to the site of hydrolysis or to the intracellular space. Furthermore, the observation, that mucosal specimens with unmeasurable lactase activity can nevertheless oxidize considerable lactose (20-50% of normal) suggests that perhaps a hitherto unrecognized mode of lactose metabolism may exist which does not require initial hydrolysis of the lactose.