The normal gross and microscopic anatomy of the palatine tonsil of the rabbit was observed, and following the direct injection of antigenic substance, structural changes were noted in the crypt epithelium. A cold light laryngoscope tube was used to inject ovalbumin or bovine serum albumin, plus Freund's complete adjuvant, into the subepithelial lymphatic tissue. Five weeks later subcutaneous challenge injections of the same protein produced increased numbers and proportions of infiltrated small lymphocytes and medium-sized lymphocytes containing a highly organized granular endoplasmic reticulum. These cells occupied wide intercellular passageways. Epithelial plasma membranes that faced these passageways remained smooth, but other surfaces of the same epithelial cells acquired vastly increased numbers of microvilli. The surfaces of other epithelial cells that faced each other also showed microvilli. These microvilli faced expanded interfacial canals. Increased numbers of small lymphocytes were observed emigrating through postcapillary venules immediately beneath the epithelium. The microvilli and other fine structures of tonsillar crypt epithelial cells are compared with similar structures of epithelial cells of the thymus. The experimentally induced increase in microvilli suggests the possibility that tonsillar crypt epithelial cells make a secretory contribution to local immune reactions.