The activity for ATP-ase was investigated in cells of rabbit and guinea pig epidermis and human oral mucosa. Observations both in the light- and electron microscope indicate that the ATP-ase positive cells of guinea pig and human epithelia are Langerhans cells while in the rabbit epidermis the reacting cells have the features of keratinocytes. A calcium containing mixture of paraformaldehyde and glutaraldehyde is recommended as the fixative of choice for demonstration of ATP-ase activity in Langerhans cells.