Summary An examination of the udder of a lactating cow by histological means and suitable staining showed the presence of cells beneath the secretary epithelium of the alveoli and in the interlobular spaces which had the appearance and staining properties of smooth muscle fibers. These cells were not observed to form a continuous band around the individual alveoli but were spaced at intervals below the epithelial cells. They are believed to surround the alveoli and to aid in the expulsion of milk from the lumen and not to be associated with blood capillaries. The walls of the duct system also contained similar smooth muscle cells but no tendency was observed for these cells to form muscular sphincters which would, upon contraction, impede the flow of milk.