AbstractWe previously reported that α‐lactalbumin combines with the lysosomal membrane of mammary epithelial cells and that it acts to release lysosomal enzymes. However, the details of this combination within the cells remained undetermined. We now report that125I‐bovine‐α‐lactalbumin in the medium entered mouse mammary epithelial cells, and about 13 per cent of the α‐lactalbumin that entered the cell bound to lysosomes. About 75 per cent of the α‐lactalbumin that reached the lysosome was tightly bound to the lysosomal membrane. It appears that α‐lactalbumin in the secretory vesicles does not migrate out, because murine and bovine whey did not induce the release of Gol