The mast cells of the skin of the external ear of the rat have been examined, with a view to the possible identification of different cell-types. It was not possible to discern more than one type of cell using criteria of size, shape, nuclear chromophilia or sizes of granules. The cytoplasmic granules of all of the cutaneous mast cells were tingible with Alcian blue (pH. 1.0 and 2.5) and, metachromatically, with toluidine blue 0. These properties were attributable to the heparin contained in the granules. The cells all gave positive reactions with the diazosafranine method, possibly by virtue of their content of serotonin. About two thirds of the mast cells in the dermis contained granules which were stainable by the periodic acid-Schiff (PAS) method and which could bind concanavalin A. It is suggested that these latter cells contain, in addition to heparin, a neutral mucosubstance characterized by a high content of alpha-d-glucosyl and/or alpha-d-mannosyl monosaccharide residues. The PAS-positive cells were relatively more abundant in vascular adventitiae and nerves and less so in the general connective tissue of the dermis than were the mast cells which did not contain the second mucosubstance.