Mortalities due to digestive disturbances occur in homozygous grey and white lambs after they have reached weaning age. Milk-filled, distended rumens, due to malfunctioning of the oesophageal groove, are found 24 h after birth. Scanning electron microscopical studies revealed that milk caused sloughing of the luminal cells in the forestomachs of the affected lambs, while no sloughing of cells was apparent in control black lambs. The purpose of this study was to compare the ultrastructure of the forestomach mucosa of grey, white and black Karakul lambs; to determine whether the sloughing of luminal cells was evident in sections; and, if possible, to find a reason for the desquamation of the cells. Samples of the forestomach of grey, white and black Karakul lambs were prepared routinely for electron microscopy and studied with a Phillips electron microscope. In all the lambs the mucosa of the forestomach was a stratified squamous epithelium consisting of a stratum basale, stratum spinosum and stratum corneum. In the grey and white lambs the luminal cells of the stratum corneum were electron dense, non-nucleated and vacuolated. Sloughing of luminal cells was observed. In the black lambs no sloughing of cells was evident and the luminal cells were moderately electron-dense, nucleated elements. Desquamation of the luminal cells in the affected lambs revealed the underlying layer with its exposed desmosomal attachment sites. This explained the differences in the appearance of the luminal cells in the three groups of lambs as revealed by the scanning electron microscope.