Mast cells are widely distributed in various organs. Two classes of mast cells, the mucosal mast cell (MMC) and the connective tissue mast cell (CTMC), have been shown to exist in the intestine of experimental animals. In the present study, we investigated the method of staining suitable for observing the mast cells distributing in the nasal mucosa, and also examined by the use of two fixatives whether the mast cells have properties of MMC or those of CTMC. A neutral buffered formalin solution and Carnoy's solution were used as fixative. For staining, five solutions, i.e., toluidine blue (TB) solution (pH 0.5, 2.5, and 4.0), 0.4 M MgCl2-alcian blue (AB) solution, and naphthol AS-D chloracetate (NAS-DC) solution, were tested. In the specimen fixed with Carnoy's fixative, staining with pH 0.5 TB showed the largest number of mast cells in all mucosal layers, particularly in the epithelial layer. The number of these mast cells agreed with that of the cells positive to pH 0.5 AB and also with that of the tryptase-positive cells stained immunohistochemically with a mouse monoclonal antitryptase antibody. Compared with formalin-fixed specimens, those fixed with Carnoy's fixative and stained with pH 0.5 TB showed significantly (p less than 0.01) many mast cells in the epithelial layer and in the subepithelial layer of lamina propria. To identify mast cells in the nasal mucosa with nasal allergy, fixation with Carnoy's fixative and staining with pH 0.5 TB were found to be most effective and simplest.(ABSTRACT TRUNCATED AT 250 WORDS)