AbstractIn the early stage of mammary gland involution, biochemically detectable lysosomal damage occurs. The mechanism(s) underlying this damage is not well understood. We found that α‐lactalbumin from mouse milk induced the release of enzymes from the lysosomes of mouse mammary epithelial cellsin vitro, and this induction also occurred with bovine α‐lactalbumin. This enzyme release was accelerated by the addition of whey proteins with a molecular weight of 50 000 to 60 000. We also found that the lysosomal membrane of mammary epithelial cells had a strong affinity for α‐lac