Mast cells in the mucosa of the gastro-intestinal tract are called mucosal mast cells. These cells are distinguished from connective tissue mast cells in the skin and serosal cavities in terms of their ultrastructural morphology, histochemistry, and responses to histamine liberators and growth factors (2, 3, 6, 7, 11) . The stainability of mucosal mast cells are well preserved after fixation in Carnoy's fixative or basic lead acetate, but not after formalin fixation (6, 16) . The mucosal mast

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... lls are stained by alcian blue, but not by safranin or berberine (10, 16), which stains connective tissue mast cells. Recently, avidin-biotin-peroxidase complex (ABC) has been reported to bind nonspecifically to connective tissue mast cells (1, 4, 15) . We have observed that ABC consistently bound to skin mast cells with sharp contrast, but not to mucosal mast cells in rat small intestine (1). On the other hand, alcian blue pH 1.0 or 0.5 stains both skin and mucosal mast cells. To differentiate both types of mast cells, we have attempted to stain these cells using both the alcian blue and ABC method, and have found that an alcian blue-PAS-ABC consecutive staining gave satisfactory results. The PAS method was used as a counter stain. Six male SD rats, 7-10 weeks old, were anesthetized and then sacrificed by exsanguination. Skin from groin, ear, ileum, and bone marrow from femur were dissected out from the animals and fixed in Carnoy's fluid. These tissues were embedded in paraffin and cut at 5µm. The three staining methods were carried out as follows : Alcian blue method : 1. Place in 1 % alcian blue 8GX (J. T. Baker Chemical Co.) in 0.1 N HCl (pH 1.0) or 0.7 N HCl (pH 0.5) for 30 min. 2. Rinse in several changes of 0.1 N HCI. PAS method: 1. Place in 1 % periodic acid for 10 min. 2. Rinse in running tap water for 5 min. 3. Place in Schiff's solution for 10 min. 4. Place in running tap water for 10 min. ABC method: 1. Place in 3 % H2O2 for 5 min. 2. Rinse in three changes of 0.05 M Tris buffer solution September 1, 1986 NAOKI ARIZONO, OSAMU KORETO, YASUHIRO IWAI, RYOJI KUSHIMA, SUSUMU NAKAO AND OSAMU TAKEOKA