Lactose synthase (a complex of beta1,4-galactosyltransferase and alpha-lactalbumin) forms lactose in the Golgi complex of mammary epithelial cells. To determine whether alpha-lactalbumin is a limiting component in this complex, transgenic mice that expressed bovine alpha-lactalbumin were studied. Transgenic mice produced 0.5 to 1.5 mg/ml of bovine alpha-lactalbumin in their milk, 5- to 15-fold more alpha-lactalbumin than in milk of control mice. Transgenic and control mice produced milk with the same concentrations of lactose, cream, and total solids, and showed similar mammary gland growth, morphology, and histology. Milk from transgenic mice had 0.6% less protein than milk from control mice (P < 0.05). The in vitro lactose synthase activity in mammary gland homogenates from alpha-lactalbumin transgenic mice was increased (P < 0.05), demonstrating that bovine alpha-lactalbumin could interact with murine beta1,4-galactosyltransferase. Pups reared by lactating transgenic mice showed a 4% increase in growth on d 10 of lactation, suggesting that milk production was increased (P = 0.06). Milk volume, estimated using the weigh-suckle-weigh technique, tended to be higher (although not significantly) in transgenic mice (P = 0.11). These results suggest that augmenting alpha-lactalbumin expression in the dam increases the growth of suckling offspring.