1. 1. A lactase-phlorizin hydrolase complex has been purified from the intestinal mucosa of neonatal rats. Polyacrylamide gel electrophoresis of "pure" lactase-phlorizin hydrolase reveals four isoenzymes, all having similar enzymatic properties. 2. 2. Lactase-phlorizin hydrolase is a glycoprotein containing 17% carbohydrate, consisting of galactose, mannose, fucose, and N-acetylglucosamine. 3. 3. At neutral pH and 37 °C, purified lactase-phlorizin hydrolase rapidly loses its ability to hydrolyze lactose. Loss of lactase activity is prevented by p-chloromercuribenzoate. Lactase-phlorizin hydrolase still attached to its brush border membrane is relatively stable at 37 °C. 4. 4. The lactase-phlorizin hydrolase complex contains two distinct enzymatic active sites, one of which (lactase) hydrolyzes β-glucosides excepting phlorizin, whereas the other (phlorizin hydrolase) is specific for phlorizin alone. 5. 5. The developmental patterns of lactase and phlorizin hydrolase specific activities differ. Lactase declines after birth, whereas phlorizin hydrolase activity is maximum at 15 days after birth. 6. 6. These data suggest that lactase is truly a β-glycosidase, and that phlorizin hydrolase may represent an active site which is more specific for the phloretin portion of the substrate than for the glucose.