Mannosidosis of Angus calves is an inherited lethal disease associated with a deficiency of the lysosomal enzyme alpha=mannosidase. It is characterised by vacuolation of neurons, macrophages, fixed reticuloendothelial cells, exocrine epithelial cells and to a lesser extent a wide variety of other cells. Vacuoles are shown to be membrane-bound vesicles and are considered to be secondary lysosomes in which are stored mannose-containing oligosaccharides that result from incomplete degradation of glycoproteins. Such glycoproteins may enter the lysomal system by heterophagy, autophagy or crinophagy. The presence in the pancreas of zymogen granules in storage vesicles indicates that incomplete degradation of secretory granules might help explain the widespread vacuolation of other exocrine cells. It is suggested that such granules may enter the lysosomal system by crinophagy.