Mucosal surfaces are protected by several complex defense mechanisms, namely mucociliary clearance, immunoglobulins, cellular components, and antibacterial secretory enzymes. The secretory enzyme, lyozyme, has been found in various mucosal tissue, and it has been suggested that this enzyme contibuted to the defense against local mucosal infections. The purpose of this study was to investigate the localization of lysozyme-secreting cells in the eustachian tube of mongolian gerbils and to provide a morphological basis for the future studies of the middle ear and eustachian tube using mongolian gerbils. In the present study, the distribution of the lysozyme-secreting cells in the eustachian tube of the normal mongolian gerbils was studied using an histochemical and immunohistochemical techniques. The results are as follows : 1) Lysozyme was produced by some secreting cells in the tympanic portion of epithelium of the eustachian tube of normal mongolian gerbils. 2) Lysozyme was prodeced by serous cells of subepithelial glands of the eustachian tube of normal mongolian gerbils. These results are consistent with the concept that secretion of antibacterial enzymes is an integral part of the normal mucosal defense system in the tubotympanum.